Brand new MySpace CEO Owen Van Natta started things off Monday with his first day on the job. He was joined by new COO and former AOL exec Mike Jones and new chief product officer and former Sling Media exec Jason Hirschhorn. Down in Los Angeles at the AlwaysOn OnHollywood conference, Boomtown ran smack into Huff Post mastermind Arianna Huffington, who extolled the virtues and abilities of new managing editor Jai Singh, former editor-in-chief of CNET Networks. At AOL, in preparation for spinning off the Time Warner (TWX) Online unit, new CEO Tim Armstrong began appointing new senior execs and spinning off existing ones. Platform-A president and former Yahoo (YHOO) sales exec Greg Coleman, who joined the AOL team in February, is leaving the company, to be replaced by Jeff Levick, who is leaving Google (GOOG)–where he had a close relationship with Armstrong. CFO Nisha Kumar is also leaving AOL, and a search is underway for her replacement. MediaMemo has more on Time Warner’s decision to spin off AOL. A number of Flickr engineers were laid off Wednesday, but Chief Architect Cal Henderson has left the company of his own accord and is working on a stealth start-up with Flickr co-founder Stewart Butterfield. Last, but not least, one of the voices covering the digital scene has found a new gig. Owen Thomas, self-described “scourge of [Silicon] Valley,” is leaving Valleywag to head up GE (GE) unit NBC Universal’s new “Bay Area” Web site, whose motto is “Locals Only.” He’ll be replaced by fresh-faced Ryan Tate, recently the night editor for Gawker. Both reporters talked to BoomTown on Friday about the changes.

MediaMemo wrote on Monday about Condé Nast shutting down Portfolio–both the print magazine and the accompanying Web site. On a cautionary note, MM outlined the reasons why Portfolio’s business magazine peers should not celebrate the loss of a competitor, even (or especially) during tough economic times. Is the meteoric ascension of Twitter flattening out? According to a Nielsen Online study, 60 percent of Twitter’s users leave after a month. This was met with a lot of skepticism so Nielsen ran the numbers again with the same results–and this time it’s sticking with them. MediaMemo also had an explanation for why the long-awaited deal between Disney (DIS) and Hulu took months and months and millions of dollars to finally come together. Digital Daily had more on that story.

Digital Daily also had more info on the ever-evolving Palm (PALM) Pre story. First, a rumor that Palm plans to launch the handset on June 7–which would be crazy, given the fact that June 8 is both the first day of Apple’s (AAPL) Worldwide Developers Conference and the day that those in the know expect the next-generation iPhone to drop. Then, there’s an assertion by Collins Stewart analyst Ashok Kumar based on supply chain research that Palm has greatly reduced its production numbers. Time will have to tell, though, because Palm certainly isn’t talking yet. Of course, things could be worse. Dell (DELL) hasn’t even solidified plans for its rumored smartphone, and already, no one really cares.

Dell’s new Adamo laptop and Studio One 19 desktop aren’t causing much excitement either. In this week’s Personal Technology column, Walt Mossberg reports that although both machines look good and function well, neither is groundbreaking. In Mossberg’s Mailbox, Walt answered readers’ questions about improving performance on a PC, using peripheral devices with an iPhone and installing Apple’s OS X on a Windows machine. And in this week’s Mossberg Solution, Katie Boehret tested three apps from the iTunes App Store that make it possible for the iPod touch to function like an iPhone.

More next week.

Yesterday, several Flickr engineers posted news of their layoffs from the Yahoo photo-sharing unit on Twitter, which caused GigaOm’s Om Malik to notice that Flickr architect Cal Henderson (pictured here) was also no longer on its About page.

According to several sources I spoke to, Henderson was actually not laid off at Yahoo (YHOO), but is leaving to start a new company–in the social-gaming arena, I am told–with Flickr co-founder Stewart Butterfield.

He and several other of the original core development team for Flickr transitioned out or have been transitioning out for quite some time, sources said.

Henderson’s Web site, called iamcal.com, still says he is the head of engineering at Flickr.

Butterfield (pictured here), as well as another co-founder, Caterina Fake, left Yahoo last year. Yahoo had paid $35 million to acquire Flickr in 2005.

Fake joined another start-up, called Hunch, last summer as chief product officer. Hunch is an answers site and is now in beta testing.