Google Plus - as in, the actual plus.google.com website and app, the Facebook competitor - has basically failed in this regard. The people on it love it! I've been on it since it started. But the people on Buzz loved that too, before Google unceremoniously shot it through the head.

Basically, the existing user base doesn't matter. So they might as well go straight to forcing people onto it, since they failed to lure them onto it.