The fact remains that facebook are doing it business wise since they have ads and promotions which is paid for,  Facebook also has recently launch the marketplace. And since Facebook is the most popular and widely used by people, it is a great tool to spread product like the ICOs. The ICOs, instead of paying Facebook to make their advertisement has resulted into bounties, which I guess is one of the major reasons facebook is banning ICOs considering how much the are likely to make if those adverts comes through them directly.