Sometimes Facebook will also show a 'slow down' message.
This is how we understand it (however we do not know for sure as we are not Facebook)
Facebook has many complicated systems for suspecting accounts of spam. The only real way we can avoid this is to act as much like humans as possible (and not like a spam bot)
Once you get banned you get locked out. We think this means you account has been flagged and means you are more likely to get flagged in future. We think each time the ban happens you get locked out for longer and longer. We think it may even eventually lead to your whole Facebook account (your personal one) being banned for life (please note we have not seen this
Our current advice
Every 10 to 20 messages do something different on Facebook and enjoy your newsfeed for a minute
Do under 50 messages per hour
Do under 300 messages per day
Take regular breaks
If you have been banned leave it as long as you can before starting again
If you see a ''slow down'' message, listen to this and stop messaging for an hour. Then resume and stick to 50 messages in an hour. If you get a 'block' do not attempt to message for 24hours.