I have only heard a few things about Google+, but from what I know about Google, my personal opinion is that...I don't like most of their designs. I mean, the search engine is magnificent! But I can't stand the email. I don't think Google+ will ever be on top, it's not that popular, definitely not popular enough to get larger than Facebook in less than a year. It needs to gain lots of positive recognition.
Facebook will probably stay on top. Unless someone comes out with something fabulous, and much better than Facebook. Remember when Myspace was the "big thing"? well, now it's Facebook, we don't know how much longer it'll stay on top, but I'm pretty sure Google+ doesn't have the odds to be on top..this year at least, or maybe ever.
But keep in mind that also it needs a catchy name. Think about it, 'Facebook', that's catchy, right? It actually is pretty unusual but catchy. Now compare it to 'Google+'(Google plus), that's not too catchy. Think about big websites, like 'Twitter', 'Youtube', 'Yahoo', or even 'Myspace'. They're pretty catchy. I'm not saying that Google isn't a catchy name, believe me it is, but Google+ is pretty much the same name as the search engine..Just with a "+".
To conclude, I believe that Facebook has the bigger chances of staying on top. While Google has to make it's way to the top.