I asked a tween relative whether they or anyone they know uses FB. They answered, "No one." They said they all use Instagram, though. So the current tween generation is still on an FB platform, just not FB itself.
I don't see Facebook going away. It's such a good tool for organizing groups of friends and getting back in touch with someone you haven't seen in awhile. Tweens aren't using it because they don't need those features, they just want to share things. Once they start to grow up and move away from their hometowns, I think they would start using Facebook unless another platform exists in the same space (which as far as I know doesn't).
One of the reasons I feel the governments should look into Facebook acquiring future competitors that it feels will be competitors. In the last few years it has either acquired or copied growing social apps that it see will be a future threat.