To me, the only truly viable way for Google to combat that is to create a truly open, federated collection of social networks that can interoperate, by releasing the underlying code to G+ the way they did with Wave, or the way email works.
It would enable Diaspora, identica, etc. to become full-fledged social networks, and all still be viable because they could talk to each other, as well as talk to G+. Don't trust Google or any other company? Host your own Plus server. Companies could host their own internal Plus platforms.
I can't imagine that Google can fight the closed platform of Facebook with another closed platform, and the open web has always been its symbiotic ally.