I think their ad ban is to avoid being sued by people who lose money to one of the many scam ICOs.
I think Facebook could implement a currency which would be adopted by a large portion of its users. If they integrated it to their site and made it simplistic at the front end it would likely launch in top 5 of cryptos. At present crypto is too complex for mainstream use. It is used by geeks and speculators (who never take it off of exchanges). If Facebook (or similar) could integrate a currency they control, to their platform, I think it would be widely used. Imagine sending your niece money for her birthday, pay local bills (gardener etc). 2 billion people have a Facebook account so no need to create new accounts etc. Massive benefits.
I think the only reason they haven’t done it yet is because of legislation. They will want to thrash out all the legal issues first.
I would love to look into this in more depth but I just don’t have the time unfortunately.