Facebook may have partnered with AIM to bring chat to your desktop, but if you're not a fan of the official AIM client, there's now an easy way to get Facebook Chat in the IM client of your choice.
Over a year ago, Facebook announced that they planned to support Facebook Chat over XMPP/Jabber, the same protocol that Google Talk uses, so people could use Facebook Chat in their favorite IM client—and then it was never mentioned again. Yesterday, though, Facebook announced that in addition to the AIM partnership, the XMPP/Jabber implementation had finally come to fruition.
If you are using a multi-protocol IM client such as Pidign, Adium, or iChat, set-up is really easy—you just add a new Jabber or XMPP account, the username being your email@example.com, with your password being the same as your Facebook password. If you still haven't gotten a Facebook username (the tag that appears after www.facebook.com/ for your profile page) you'll need to go to your Settings page to enable it.
Once you log in, all your Facebook friends should show up in your buddy list. If this is good enough for you, that's fine, but I found it really messed with my organization. Although Facebook lets you decide which friend lists do and do not show up in your IM client, it doesn't seem to realize that those not on a list are important too—so if you have friends that aren't on a friend list, they'll just show up in a list called "Buddies" on your IM client, and there's no way to take them off (short of hiding that list client-side). Since most of my AIM and Gtalk contacts were already on a list called "Buddies", I had to create a new list for them to get my organization back on track. You may or may not care—it's just a warning that that list will likely be where all your unlisted Facebook friends go (at least it was for me).
Apart from that, everything else should be handled pretty well client-side. Profile thumbs are grabbed, Away in your client corresponds to Idle on Facebook, and, if you were previously using the unofficial add-on for adding Facebook Chat to Adium or Pidgin, you should find that the official route is far less flaky as far as connection goes. The service is pretty new, so there are bound to be a few bugs or hidden workarounds—if you find any, be sure to let us know in the comments!