This is such a great article from a Googler on why Google+ might (already) be failing.
The way I see it, you really only have two options:
1) Insist on building a product that people need (and love) before they know that they needed it. See the future, take a stance, and risk failure. It’s such an impossible thing to do, that when someone actually repeated does it, it leaves little doubt why Apple is where it is today.
2) Build a platform in which everyone could excel. Ignore the fact that most people won’t put in the effort, assume nothing and leave all the possibilities open.