If the purpose of a social network is to connect with other people, you need a social network that has plenty of members. And Facebook definitely has plenty of members, despite all the negative publicity it has received lately about its privacy problems. Will all 500 million Facebook users migrate to Google Me? I doubt it -- and those 500 million members are what makes Facebook so attractive.
You may argue that those people who abandoned Facebook because of its privacy problems might make the move to Google Me, but unless they're bringing millions of their closest friends along with them, they're not going to do much good. And Google doesn't exactly have a stellar reputation when it comes to social networks and privacy (see the aforementioned section on Google Buzz), so it may have trouble winning people over there.