Honestly with the Android craze, Google+ maybe the closest thing to Facebook in a couple of years. But even if every resource was spent to make G+ so fun, it can only get as high as Facebook, not overturn it!!
The task to be as popular as Facebook will take more effort too. Yea soon Google will dominate the whole world with Droid Devices, and yea you cant even sell that right with no Facebook...
So of all the social networks, I'd say G+ has the best chance getting close to Facebook in terms of use and popularity...