I personally believe that Facebook is the World of Warcraft of Social Networking. I'm not a fan of Google +, no matter how much I've tried to be, so I know I won't be switching; however, I don't see it having a problem adding the final blows to the MySpace deterioration that we've seen over the past few years.
The difference between Google + and Facebook, though, is that Facebook's trying to implement itself into every aspect of your personal life, I don't see Google + doing that.
You know how Myspace has a bunch of scene and emo kids on it now?
Well, Facebook will become like that, except it'll have all the parents / older people on it, and everyone below 55 will switch over to G+.
Eventually, G+ will become the common site. People are STILL scrambling for invites..
Facebook killed by Google? I doubt that.. Facebook is gaining popularity on the net.. but still Google is a famous search engine and used mostly for surfing by like million, trillion people worldwide and Facebook, well not all people have Facebook.. So my guess Google wins over Facebook.. :P