I wonder if Google will ever make a separate address book application to be shared across their various services. I think it makes sense. Identity data is a generic and widely-applicable information source. When I respond to an email about a party at a friend's home, schedule an event in my calendar then call to check some more details on the day, those actions all involve contact details belonging to my friend. I guess in a way what I'm talking about is a social network tool, but less focused on finding and ranking my friends than maintaining my address book. I'll have to see what Google's Orkut is like one of these days.
Let's say Google maintained a central address book of contact details and each person was responsible for keeping their own details up to date. That's a lot like Facebook, but I imagine more granular control over who can see what. Define various groups of people and define what those groups may access. Work contacts can see my work address and work phone. Family can see all my phone numbers, postal addresses and email addresses. My crazy ex co-worker is only allowed to see my name.
I don't think Facebook will last forever, whether it gets too bogged down with applications or just your creepy ex-co-workers. In the second case, I think having more control over who can see what would help alleviate the problem, at least for a little while.
Mokalus of Borg
PS - Privacy concerns can force many issues.
PPS - See this article for some similar thoughts.