> I want my search results to be ranked with signals from other relevant people... and I don't know who these people are.

This is were you're partially wrong. Personally I know many people that I'd like to be included in my search ranking as signals. The problem is - those people are only a subset of the people that I follow on Twitter or Google+.

The problem with "Follow" or "Add as Friend" or whatever these social networks are calling it - is that some people are more interesting than others. Some people I follow simply because they are my friends. While other people I follow because I'm interested in whatever they've got to say. And this is the problem that Google should solve.