Facebook has more than 83 million illegitimate accounts
http://www.bbc.com/news/technology-19093078
According to the Yahoo News version of the story, Facebook claims that the vast majority of the "illegitimate" accounts fall into two categories:
1) Members who forgot their password or other login ID and set up a new account to get back in. I won't go anywhere near FB, so don't know if they've got the usual "forgot my password" thing which would presumably avoid this.
2) Commercial (including non-profit) accounts which should have been set up as pages rather than accounts.
While they agree that in some cases an account might have been set up specifically for spamming/scams, they feel those are a very small percentage.