I think it will stay
The user experience isn't just 'better'. Its awesome. With Google+ Hangouts offering extra functionality in the whole collaboration initiative, and perhaps integrating even more (and better) with Google Apps in the near future, I think it will stay established. I don't believe it will catch up to Facebook anytime soon, though.
I think Google+ will be to Facebook what Apple Macs are to PC's, in terms of numbers and differentiating strategy.