Advertising your site as a "Facebook competitor" (instead of implying it) is a recipe for failure because you want people to look at your site as something completely unique rather than putting it next to Facebook immediately.
I think that's true for small startups, but for someone like Google? Presumably, nearly everyone with a gmail address will already be a user, which means that there won't be any migration hassles - "I'd join, but Timmy and Jimbo aren't on GooBook yet!"
But I understand what point you're trying to make. This will get used, but I don't see it becoming a true Facebook rival because of the network effect Facebook relies on. If users were to migrate in masses, on the other hand, Facebook would be dead within a short amount of time. Sadly, that's not going to happen anytime soon.