It's not that fun to have to go through all of your friends' userinfo pages separately to gather this information, especially not multiple times if you want to try a new service or lost your contact list, etc. Not to mention, it's hard keeping track of anyone with new information, or your new friends' information. A central page (or pages, if a different one for each service) with all of this information, possibly able to be sorted by the date the friend was added or the information was updated (if LJ even stores that information). I think this feature would be REALLY useful if it isn't already implemented in some way.. it would save a lot of time, hits, and just plain be cool. Like birthdays.bml.
Sorry if this has already been brought up, and I missed it going back quite a few pages-worth of past community posts. :)