Oh, okay! I just found it myself, was gonna tell you...
Actually, for anyone else who has trouble finding it:
Go to where you edit your profile. You'll see the ICQ, Yahoo, etc. stuff that was mentioned above.
Keep scrolling down. You'll see a box where you're asked what shows you like. Scroll down past that.
Below that, there are spaces to add your Facebook, MySpace, and Twitter user names. Plug the appropriate thing into the appropriate place.
Hope this helps someone!