Furthermore, I think the next popular network would bill itself as being anti-FB.
They can try to claim that, but any business that stores people's photos, personal profiles, posts, messages, etc. would have everything it needed to become just as bad for privacy as Facebook if they wanted to. They would need to convince people that they would not do bad things with the data. But even if we trusted them, they could change their minds at any time, or hackers could steal the data.
The only way to prevent such data from being stolen is to never collect it -- but a social network without that kind of data would not be very social.
A new social network doesn't have to be advertising-focused or based.
One of the ones I'm looking at is taking registrations while in closed beta. I think it is to generate a userbase who can all begin to access it at the same time when invites are sent - to hopefully gain a large amount of popularity quickly.
Facebook knows that a competitor could pivot to replace facebook, so they purchase the others.