Facebook has a strict policy against advertising online casinos, that's why you can hardly see any adverts about any casino website when chatting, however you can still advertise an online casino as long as you can provide a license for that Casino website.
Once you provide the valid licence, your advert will be approved and viewable to any country or set of people you're Targeting to see it, Facebook has many users, so I suggest you target a region where the online casino deposit and withdrawal methods are supported.
Once you provide the valid licence, your advert will be approved and viewable to any country or set of people you're Targeting to see it, Facebook has many users, so I suggest you target a region where the online casino deposit and withdrawal methods are supported.