A coworker and I recently set up a fan page for our library in Facebook, MeeboMe, and Pidgin.
The fan page was a lot of fun to create, as well as quick and easy. We suggested to a number of student workers that they become fans. We are currently up to 11 fans. Our fan page includes a MeeboMe widget, as well as the usual discussion board, pictures, notes, events, general info, etc. I may have to peruse the list of apps to see if any others would be good additions. Also great about the Facebook fan page is that you can see statistics. People may become fans and never look at the page again. Stats allow us to see how often people actually visit.
We also put a MeeboMe widget on our subject guides (hooray for pbwiki's sidebar! I did not want to have to install it on each page. Plus, if you edit the page, it changes the widget code for some unknown reason. This makes the widget stop working until I re-paste in the correct code. Not sure why it does that, but it's quite annoying). We also intend to find out if it's possible to put the widget on our library page within the campus portal.
Trillian, our current IM client, only allows us to add AIM, ICQ, MSN, and Yahoo accounts in the free version (how silly). So we switched to Pidgin. Chad Boeninger at Library Voice has an excellent explanation of how to set up MeeboMe and Pidgin. We also added a Google Talk account just to cover all our bases. Not that students don't all just use AIM anyway.