No one thing will usurp it, but it will be multiple things. Maybe dozens.
How much bigger can Facebook get? By now 99% of people who want to be on it are already on it.
International markets, especially Asia and Africa. More than half the world's population is there and there are a lot of potential users. I agree that the user base in North American and Europe has been almost completely tapped, but there's a lot of growth left if Facebook plays it right.