+1 to goliasd89293.
Just because they're on a social network doesn't automatically mean they're using it to stalk victims. If they could actually find evidence this was true, it would make sense, but it sounds like they instead just decided that because they were a registered sex offender, they'd kick them.
Oh, and by the way, not all registered sex offenders are as bad as the ones described. You can get registered as a sex offender these days for something like an office 'joke' and patting some girl on the behind, if they take it seriously enough. Sure, it may not actually be funny, but it can get you on the list, even though it's a 'minor' infraction.