That is a very interesting phenomenon to think about. The problem I see is that I think it's going to be hard for people to stay on any single social network for all that long. It seems like Facebook is sort of already just a bit over it's hump and is slowly going downwards. It'll be around for awhile longer - no doubt - but it's hard to say how much longer. But obviously there will be at least some subset of people that continue using Facebook for many, many years to come, and maybe that's all future historians/anthropologists will need is a few interesting cases that they can explore (although obviously more would be better).

I'm not exactly sure why, but I see Twitter as having a bit more longevity.

EDIT: This sparked a great conversation between a friend and I on this topic (thanks!), and he thinks that it'll cost too much in storage space for Facebook (et al.) to move dead profiles over to new storage before their drives start failing. Is it worth it for these companies to keep old, unupdated profiles around? Obviously famous ones can be kept easily enough, but what about others? Where's the line drawn? Will the kickback from families not wanting their grandparents Facebook timelines lost be enough to keep the information alive? Any thoughts?