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The killer advantage that he didn't mention is that Google search is used by a billion people worldwide, and is probably the start page for at least half of them. You still have to go out of you way to visit Facebook, by explicitly selecting it from your bookmarks, but seeing your Google+ notifications will be automatic and unavoidable.

Sure, some people will turn the feature off entirely or simply not have any friends to bootstrap themselves into the network, but hundreds of millions of people will be in a position where their interaction with Google+ will be automatic, and if you're already interacting with friends there, why bother going to Facebook?

Quite simply, a lot of people will be on both for a while, and use Google+ more and Facebook less over time.