G+ has its set of users and it might grow in the coming days, but by integrating G+ with Google search they are destroying the sanctity of search. Having to sign up for a social network so that my blog posts can have my photo and name appear in search results is evil. Google can use existing microdata to achieve a similar result. But they want to push their social network down my throat if I want to make my website appear in google search with authorship data.
I haven't posted anything on Google+ is well over a year. I don't think people moving to Google+ from FB. And, I'm sure Google+ is not the next big social network.
I hope that the next social network will be real life and that online services will go back to what those things used to be: websites that do one thing good (see Flickr) rather than trying to buy our mortal souls. Or giving me (us?) the feeling this is is what they're trying to do.
