Until now I have not seen Facebook will prohibit or restrict cryptocurrency. And I really don't understand if Facebook forbids it. There are rumors that Facebook will issue its own currency, I think it's just a discourse. Not necessarily realized.
Yes i read that Facebook banned advertisements about cryptocurrency but they changed their decision.
I also read that Facebook is "very serious" about launching a Crypto
https://cryptobriefing.com/ready-for-facebook-coin/