Over the summer, Spotify finally opened for business in the U.S. and included a free, ad-supported option in its offerings. Now both MOG and Rdio, two other subscription music services, are adding big free components themselves.

Tomorrow MOG is rolling out a Web-based, ad-supported version of the service that gives users an undisclosed amount of free streaming music, which they can keep listening to if they engage with the service in certain ways, like sharing playlists with friends.

And soon Rdio will offering something similar, with two differences: Its free version will be ad-free, and the company won’t prompt users to take certain actions to keep the free going. It will decide, out of the user’s sight, how long to extend the free trial period, with the intent of getting them to upgrade to a $10 monthly subscription.

The timing of the new free services aren’t accidental. They’re both being announced in advance of Facebook’s F8 developer conference next Thursday. That’s when the social network is expected to announce a new music service that will incorporate MOG and Rdio as well as Spotify.

Details about Facebook’s service are still sketchy, but the primary gist is that Facebook won’t be building its own music service. Instead it will work as a hub that allows existing services’ users to share their tunes with their friends.

But it’s very unlikely you’ll be able to listen to a friend’s Spotify song unless you’re a Spotify user yourself. So it’s crucial for the services to make sure that signing up to use them is as easy as possible. And requiring someone to pay to listen to a friend’s song seems like a non-starter.

Hence: Lots of free trials that last for an undefined period.

Rdio CEO Drew Larner won’t comment about Facebook and its upcoming service. But he will talk about the strategy of offering free music, but only for a limited time. “We know free is powerful, it’s a great way to increase your funnel,” he says. “But it’s about not creating a tier for an alternative or replacement for what would be a paying subscriber.”

Ultimately Rdio, MOG and Spotify, along with services like Rhapsody, are all trying to get to the same place. They want customers to pay them a $10 monthly fee for unlimited music that works on both the web and devices like Apple’s iPhone and Google’s Android.

That’s a model the music industry was never happy about until recently, in large part because free ad-supported services like iMeem, MySpace Music, Qtrax, and Spiralfrog sputtered and/or failed completely.

The other option for music fans who don’t want to pay for music but don’t want to break the law is to play with Web radio services like Pandora and Clear Channel’s new iheartradio.

Those services let you listen to unlimited music, for free, but don’t give you on-demand access. Instead, you need to let them program your music for you in some capacity.

Somewhere in between the two sits Turntable.fm, which is also free, but gives users more control of their music — for now, at least, while the company tries to hammer out label deals. The service is also expected to play a role in the F8 announcement next week.

However, even with the masses descending upon Las Vegas this week, Cricket has decided to delay the Muve music service and the launch of its first Muve-compatible phone.

Cricket had hoped to launch service in Las Vegas this week along with a number of other markets later this month.

Under its revised time frame, Cricket plans to launch the service in Las Vegas later this month, add nine more cities in February and roll it out to the rest of its markets this spring.

Although all the necessary licensing is in place, Cricket spokesman Greg Lund said that the company needed the extra time to ensure all of the software bugs were ironed out before it started asking customers to pay for the service.

“It’s got to be just perfect,” Lund told Mobilized in an interview at CES in Las Vegas.

The company, best known for its prepaid phones, is offering a new service called Muve Music, which includes the cost of unlimited music downloads as part of a $55 monthly cellphone plan that also includes unlimited talk, text and Web. Basically, Muve adds about $10 to the cost of the monthly cellphone tab (which, incidentally, is about what one can expect to pay for the typical subscription music service).

The music is downloaded directly to the cellphone and is accessible as long as you remain a subscriber. From a technology standpoint, the service works by transferring the music to a secure partition of a 4GB digital memory card in the phone; Cricket says that partition can hold about 3,000 songs.

Cricket is launching the service next month with a single compatible phone–a color touchscreen feature phone known as the Samsung Suede, which will sell for $199. The service will first be available Jan. 6 in Las Vegas, with about 10 of Cricket’s other markets due to come on board later in the month.

What makes the service interesting is the approach–there is no tie to a PC whatsoever. Music comes to the phone, lives on the phone and is managed on the phone. In an interview, Cricket Vice President Jeff Toig said the service is geared to Cricket’s base of customers, many of whom don’t have a PC and broadband connection. It also allows them to get their music the way they do their other cellphone services–by paying in cash at the company’s retail outlets, thereby eliminating the need for a credit card.

The downside, of course, is that the music can only be played on the phone, though the phone can connect to a car stereo or external speakers over bluetooth or via a 3.5mm cable.

On the plus side, Muve doesn’t add much to the cost of a cellphone plan and eliminates some of the complexity traditionally associated with digital music. In addition to the ability to download and play tracks from all four major music labels, Muve subscribers can set any track to be either a ringtone or ringback tone (the music heard by callers while they are waiting for someone to answer).

Nokia tried a somewhat similar approach with its “Comes With Music” phones, which debuted in 2008. In that program, the cost of the music subscription was included in the price of the phone rather than in the monthly cellphone bill. Others, such as SpiralFrog, have tried to create services relying on advertising to subsidize the cost of providing music free to the end user.

Toig said that his customer base is one that typically isn’t downloading music from iTunes at 99 cents a pop, but includes a fair number of people that illegally download music from file sharing services.

The new service, he said, allows them to have a better experience without having to spend much more than they already are, while giving the record industry a chance to reach digital music customers they are largely missing out on today.

Unlike other services, which Toig said bank on the fact that people have access to a computer, Muve tries to make it easy to discover and download music directly from the phone. Customers can subscribe to curated feeds of music that get automatically updated, as well as find and download albums by name. A built-in social network allows them to see what their friends are listening to (assuming they also have a Muve-compatible phone).

“Nobody has done mobile music right,” Toig said.

Cricket won’t say how much of the incremental $10 in monthly revenue it is getting goes to the labels, but Toig said part of the bet is that Cricket will be able to reach customers that it otherwise could not.

“We’re obviously not doing this for a few percent,” he said. “We think this has appeal beyond our base to segments Cricket has not appealed to before.”

Since everyone wants to weigh in on the state of online music service, imeem, let me add in my two cents: The once-buzzy start-up isn’t on life support yet. But it sure could use some help–just like every other Web music player.

I can confirm that the company has sought, and received, new terms from some of the big music labels, most notably Universal Music Group. One big label that hasn’t given imeem any concessions yet: Warner Music Group (WMG), which owns an equity stake in the company.

Wired.com has a nice summary of the reason imeem needs new terms from the labels. But the short version is that the original deals it had in place meant it lost money every time someone played a song. And that the more popular it got, the more it lost.

That kind of business plan isn’t out of the ordinary for Web companies that struck deals with the big music labels–even Google’s (GOOG) YouTube has a similar kind of arrangement. But those deals, which were supposed to make sense once the online advertising business kicked into gear, are also obviously no longer sustainable.

The big picture: imeem went looking for buyers last fall–at the same time that its venture investor Sequoia Capital began lecturing start-ups about the need to cut costs–and hasn’t found any.

That puts Imeem in the same boat as every other notable online music service, including iLike and Pandora, all of which have been looking for money or an acquirer during the last six months.

One of them–Spiralfrog–finally shut down earlier this month. And if the other start-ups can’t get the labels to cut them some deals, Spiralfrog will have company.

[UPDATE: A source familiar with the situation tells me that the company is indeed dead and that any remaining assets are being handed over to creditors.]

Music industry sources had told me that SpiralFrog, which offered free, ad-supported downloads–with several strings attached–was shopping itself in recent months.

When I asked CEO Joe Mohen about that on March 2, he insisted that his company was merely seeking “to have a strategic relationship with certain large companies to augment our sales team.” If he calls back, I’ll tell you what he has to say today.

It wouldn’t be surprising to see other music sites go dark, too. Several of SpiralFrog’s competitors have been looking for money or buyers as well.