That’s because it’s a Facebook app campaign, which makes it very difficult for its sponsor to blow much dough, no matter how hard it tries. In this case, Citi is sponsoring a “Magnetic Lyrics” app, a takeoff on  the refrigerator magnet word games favored by vaguely literate folks. In this case you assemble lyric snippets to your favorite songs by either Nickelback or Mary J. Blige.

Don’t like either act? Me either. But presumably lots of people do, and Citi is doing a broad deal with both of them. In any case, what’s relevant here are the economics, which are small scale.

I don’t have specifics, but I assume that Citi and its vendors–Atmosphere BBDO, a digital agency owned by ad behemoth Omnicom (OMC), and Buddy Media, a start-up that specializes in social media campaigns–are spending a modest amount to actually develop the app. I’m guessing it’s under six figures. And Citi will likely pay a small bounty for each user who plays with it–perhaps $1 per user. So far, that’s about $5,400.

And what does Facebook get out of this? Well, Citi (C) will need to buy some ad space to promote the app, so you can tack on a few bucks more to the campaign. But not much–Facebook has had a notoriously hard time convincing advertisers to pay up for its inventory. And it has a lot of inventory.

The takeaway: Assume Citi continues to exist in some form, it’s going to have to continue to spend money on marketing, and most of that will still be on big-blast campaigns like the ones you’re used to seeing on TV. But small, targeted plays like this could be an effective complement. And if you get a lot of marketers investing a lot of money in these kinds of gimmicks, then Facebook has a real income stream, which it could use.