I don't think they're buying it per current user. It's either about paying for future users (granted, triple the user base and it's still a lot) or a game of defense against a potential competitor.

They bought the photo company at what seemed an outrageous amount in part to co-opt a competitor. In the end it was profitable. They alter their feed to look more Twitter-like, encouraging people to avoid the competitor. I see this as defense too. It isn't about new users, it's about corporate competition.