Facebook isn't going to be going away in a hurry but Google+ will almost certainly take out a chunk of its market share over the next year or two.
One thing to remember is it fundamentally differs from Facebook in quite a big way because it is an open network, whereas Facebook is a closed network. It is because of this that has allowed the comparatively huge growth of Google+ in such a short space of time.
Personally i'd think of Google+ plus as more like Twitter but with the functionality you get with Facebook - I can see it turning into a great platform for businesses and the like to promote themselves.
*Disclaimer* These are entirely the opinions of a half wit, please take with a generous amount of salt.
