The importance of a proper address book is huge for me. Since I have started to use the Internet in 1994 (previous Amiga 500 BBS connections don't count), I kept a good accurate address book on Hotmail first, then moved this to my Outlook and Gmail.
When I open an account in Facebook, I have updated my address book with my old friends that I have found there and even tried to add a picture to each but never get around to get them all. All in all, I am proud of my address book on both Outlook and GMail.
Now the beauty of the Web2.0; when you join to web sites like Twitter, FrienFeed, Last.fm etc., you can give your Gmail or Hotmail etc. account credentials and search your friends automatically. Finding most of your friends depends on the accuracy of your address book.
I have too much accounts on the web on various web sites and meshed up this MS Visio diagram to see what goes where. This is the second version of the diagram.
Things are looking good at the moment. I still need to update my address book with the new friends on FriendFeed and Facebook.