Mass and Inertia
Don't underestimate the power of inertia. Being a bit technical like, I might think Google+ is the new dawn of happy social networking (I really don't, but let's pretend). Yay, good for me. Unfortunately many of my friends are non-techies/followers of the new and shiny and couldn't give a rats, they are perfectly happy on Facebook and so are their friends. So, unless G+ comes with a magic longevity pill (or loads of free stuff), they aren't moving, so neither am I.
Huge mass of users equals huge inertia - see Microsoft. You'd need something much more disruptive to get the mass moving. While rapid movement in the technical space isn't unheard of, in the consumer space it's a little harder to achieve. Apple managed it with the iPad, but the imitators are in a similar position to Google, unless they can offer something really, really cool, whether in function, design or price terms, they'll always be sucking on hind tit.