As you say, it is Facebook policy rather than law, but if the police tell them, then your account will be closed much faster than if you were an abusive troll.... As xDanx said, if it's not on your SHPO, it's not a breach. You are required to notify your account details, as social media usernames are considered aliases, but there's no requirement to supply passwords. After all, they probably have monitoring software or the right to check other devices, so they will know which sites you have visited and what apps you have on your devices.

If you have an SHPO in force, you are effectively on the SOR anyway (see under "Notification requirements"). As you probably all know, I'm quite a cynic in these matters, and for most offences you can be considered an ex-offender, but generally not for a sexual offence, regardless of the technicalities. As far as I'm concerned, anyone is an ex-offender as soon as they stop offending, just as an ex-smoker is someone who has stopped smoking. However, many people see it more along the lines of drug/alcohol addiction, where you can never be an ex-addict.