We're still expecting an official Facebook application come the launch of Windows Marketplace for Mobile, but we've been using the version, er, purloined from a Windows Mobile 6.5 ROM for a while now. It's good stuff.
Noir over at PPCGeeks points out that there's a new version available (0.16 - up from the 0.14 I was using) that for whatever reason is definitely a little more snappy. There's also a version at XDA Developers [via], but I had trouble with that one.
That said, this is highly unofficial and unsupported stuff (hell, the name of the app has changed from Microsoft Facebook to ppcgeeks Facebook - and that's more than a little shady), so be sure to back up your phone first, or just lurk in the thread and see how everyone else is doing.