I highly doubt that Facebook have policy about crypto bounties. It's just general policy against spam. Thwy have their algorhitms to block posts which looks like spam. If you will post these bounty links everywhere where you can, offcourse you will be banned. First you may be asked to verify identity, if you continue to spam you will get temporarry ban and later permanent ban.
I noticed that Facebook catches these spammers much better now. Now I'm not able to post some referral links on FB groups because they always blocks all these my posts.