I don't think I would underestimate Google and it is a lot better at allowing you to segment who sees what which seems kind of basic to me and I reason I never liked Facebook.
it means if someone already had a profile on google + he/she needs to add more and more users so that he can get more and more backlinks rite ?
You must use keywords while sharing links in Google+. This article of mine explains how Google+ helps in SEO http://www.blogotechblog.com/2011/07/google-plus-affects-seo-website-blog/