I don't think that Facebook will have to worry about losing users with the level of growth they experience. It's the US government that's worrying about their size and even considers splitting Facebook up, which would be insane.
I don't know how I feel about a split up, but at least it would offer an opportunity to the smaller players in the social media field to gain more ground while they right now don't stand a chance.
Remember that those who do seem to stand a chance to grow, have been and will be taken over by Facebook. It's a powerhouse that doesn't seem to slow down unless regulators start to dig into Facebook.