People leaving is not necessarily a problem, if the new ones come faster. If you are small, you can grow exponentially even if literally all users left in a month. But this becomes a problem once you grow so large that there is nowhere to grow. Which is probably the case with Twitter -- everyone already heard of it, and everyone who was tempted to try it already did.

On Facebook, I think the only reasonable way to use it is to create groups and only debate within them. Also private messages, but only 1:1.