Charles Arthur at The Guardian:
Discussing [Google+] as if it were a social network which needs activity in the way that Facebook and Twitter do misses the point. It really doesn't matter if you never use it, never fill out your profile, never fill a circle, never get added to anyone's circle. What matters to Google is that you're signed in, in order that it can form its matrix of knowledge about you.
The whole piece is definitely worth reading and The Matrix analogy, though overblown, is apt. We all know Google (and perhaps -- as The Guardian revealed today -- the US government) capture all our search and usage data across all their properties and use it to sell advertising, but we continue to use them because they're the best. The red pill (Apple Maps, for example) is a considerably less pleasant option.
However, I don't think Google needed to create a social network purely as a means to tie this data together, they could do that just fine with the single sign on they already had.
Google+ is a hedge against Facebook the same way Android was a hedge against iOS. It's just a less successful one right now.
Google can't spider any of the information on Facebook, the more user activity that happens there the less Google sees. Not to mention all the super valuable relationship data that a social network generates.
I think Google would love people to use Google+ as a social network more and are leveraging their market leading products in other areas to try to encourage it.