In most jurisdictions, I think in even when a sign like that's posted, you do at least have to ask the offender to leave before you can call the cops. If they walk into your restaurant without a shirt, you ask them to leave, and they refuse, then they're guilty of trespassing. But if they do leave when asked, you can't go ahead and press charges.
I believe an exception is if you've posted "NO TRESPASSING" signs, making it clear that it's private property to which all entry is prohibited. But if you've invited the general public in subject to conditions, and someone violates a condition, they haven't yet committed a crime, unless they also refuse to leave when you try to eject them.
In Facebook's case, I'd say they've invited the general public to use their service, subject to some conditions, and so no law should be involved unless they've specifically asked the person involved to stop using their service and the person continues anyway.
I believe an exception is if you've posted "NO TRESPASSING" signs, making it clear that it's private property to which all entry is prohibited. But if you've invited the general public in subject to conditions, and someone violates a condition, they haven't yet committed a crime, unless they also refuse to leave when you try to eject them.
In Facebook's case, I'd say they've invited the general public to use their service, subject to some conditions, and so no law should be involved unless they've specifically asked the person involved to stop using their service and the person continues anyway.