Switching costs are absolutely the thing to focus on here, but limiting the switching costs to a replication of the social graph may be understating the true cost. Think of all the photos, and to a lesser extent, wall posts. I would imagine a lot of the time that's spent on Facebook is just browsing through photos (and again, to a lesser extent, people's wall posts). Much of it probably is just new photos, so obviously those don't need to be replicated since if you port the social graph over, people will start posting photos on the new site and people will just look at those, but a ton of the viewing is probably of legacy photos. That is, some of the viewing is seeing what's happening with your friends (the party last night) but a ton of it is just checking people out (facebook stalking). How can I stalk somebody without seeing all their past photos/videos/notes/posts/etc.?