I read a description of the computer desktop as the place which you can always find on your computer, and so the place where unconfident computer users end up storing all of their files. I think Facebook is the equivalent of the desktop for social contacts on the web. Which I think will give it a lot of resilience to unpopularity, so even if no one loves it, it'll probably stick around.
A great analogy would be phone books. I've not found a person who actually likes getting or using them, but they were the best place to find someone's phone number if all you had was a name and a bit of an address.