Just a thought on the success of Google+, which announced it has 18million users: I have two FaceBook accounts. One account is for family and real-world friends, the other is for FaceBook friends. I know several people who have at least two FaceBook accounts. You can have as many FaceBook accounts as you have email accounts as I understand it. However, with Google’s strict rules on membership—members must use their actual names—I doubt that you will find users with multiple Google+ accounts, even if they have two or more gmail accounts, because the focus is on real people using their real name.
FaceBook’s 700million accounts is not an indication of 700million unique users. I have searched for statistics that show how many users have more than one account and that number does not appear, which reinforces my belief that Facebook does not want to break out that number, perhaps because it is significant. Just a guess. Certainly you should not cut that number in half to arrive at the accurate number of unique users but it could be a significant reduction from 700million. So, more weight has to be given to Google+’s 18million users. Not really a big deal.