Really, asking "what should replace Facebook" is putting things the wrong way around.

A more interesting way to ask the question is, "what did Facebook replace."

People used to build their own websites. People used to have blogs. People used USENET which was truly distributed and un-censorable.

Facebook and Google took the open internet and open standards and monetized and made everything crappy. Enough of that. Nothing should replace Facebook, it's done, stick a fork in it.