Address Book in Lion supports a number of new contact fields for social media and IM entries. You can now add usernames for services like Facebook, Twitter, Skype, and even QQ.
For bonus points: you can actually do stuff with these new social profile links. Address Book adds an @ symbol in front of Twitter user names, and you can click the Twitter field title for options to open the user’s profile in Twitter.app or create a new @ mention to them (unfortunately, Address Book is hardwired to use Twitter.app. If you don’t have it installed, Address Book falls back to your browser). Click the Facebook field title for options to open your default browser and go straight to the user’s profile or their photos.