Google is a much more successful company than Facebook, and it is likely that they will be around long after Facebook goes out of business. Although Facebook is popular now, it is not clear that this company will ever be profitable. Google, however, makes money through advertising and selling software.
Google Plus continues to grow in popularity, but it won't reach the level of Facebook anytime soon. Facebook is used by more than 100 million users around the world with billions of interactions between users each day. In the next few years, Google Plus won't even come close to matching that.
Facebook may not always be the top social networking website but it certainly won't bow out to Google +. Google + by their launch was already dead. There was a push by tech users and the kind of people that use "linkedin" trying to make it happen, but it just won't.
Google plus actually does have a lot of good things tied with it and a lot of good services. However, it will not beat a Facebook level of popularity due to that not too many people actually use Google Plus and it's easier to find people on Facebook anyway. Facebook will win.
Google Plus is not going to win out over Facebook. Facebook has been around too long to worry about a downfall by Google Plus. If Google Plus continues to grow then they will have their own set of users but I really doubt this will discourage people from using their Facebook.