The key utility of social networking sites is that allows you to stay in touch and up to date with a larger circle of associates than you could otherwise. Facebook lowered the cost of maintaining those associations by making the process more efficient. They have been steadily raising the cost with every added feature since then.

Done. It's called reddit, and unfortunately not federated.

We're in a recentralization cycle of services, because it's easier to build (and more monetizable) than similar distributed services.

When we have the technical building blocks and business creativity to break the cycle, things will get interesting again.