It always seemed to me that social is the evolution of search. And google knows this very well. Social signals are very important for finding what you're looking for. The idea of g+ was always to implement something like the new "google + your world" on a larger scale on all google products. If they didn't do this, they would fall behind to the competition. They felt forced to.

Erick Schmidt once said something like "we only want facebook to open their data so people can search for it, failing that, we'll find other ways to get it". It was clear from that day, that they were building a facebook competitor. Not because they were looking for new ways to spam people with ads (that's an overly simplistic generalization). But because they found it important to keep their existing business relevant. Independent of which monetization they were using. Even if google search was a paid premium, they would need social anyway. It's clearly not "about the ads".