"Sure, Google+ doesn't have anything close to the numbers Facebook boasts. But there's still plenty of value Google can wring from its social network and plenty of places in which it can grow, according to Google VP Vic Gundotra. Google+, he said, is a social layer that goes on top of everything else Google does, rather than an isolated destination."
I agree that Google is playing a deeper, longer-term game with Google+. This report of a chat with the site's chief indicates a bit of the plan, but I expect there's more to it.
Regardless, I don't think Google's intent is to topple Facebook, but rather to counter Facebook's lead in social search.
Via Matmi, Jekaterina Cernobrovaja