For example, Skype has omnipresent for years, but Apple's Address Book had no field to store a contact's Skype name. Now, the IM fields can be specified as: Facebook, Google Talk, Skype, Yahoo among several others.
There is also a new Profile section in Address Book, with Twitter, Facebook, LinkedIn, Flickr, MySpace and Custom labels, although there seems to be something lacking.
When I filled in Facebook with a contact's name and saved the changes, clicking on the 'Facebook' label, there were two options: 'Show Profile' and 'Show Photos.' They didn't work. But, adding profile.php?id=nnnnnnnnn where nnnnnnn is the profile number of a contact, will indeed get you to their profile.
[crarko adds: Welcome additions to Address Book. Hopefully it will continue to improve in future updates.]

