You can't just spend a bunch of money advertising on facebook because your audience won't be geared towards crypto. They will visit the site, get confused what "Bitcoin" is and just close the site.
Hence you need to do your research and find the most efficive method. Many people opened up "crypto casino review sites" and try and get them popular. If they get some organic high search engine google listings then they might get traffic and get high quality revenue that way. However even those are saturated these days.
The most cost-effective promotion method is advertise on biggest sites on markets in crypto industry:
coinmarketcap.com;
coingecko.com,
massari.io, and so on.
Starting with sites that are daily visited by crypto users who will be more familiar with bitcoin, crypto and more easily to accept crypto online casinos.
You can definitely make advertisement on facebook, there are tons of pages with crypto focus there as well. Do not think of facebook just a place where elderly hang out, even they moved to instagram these days and right now there is no place you can't see elderly hanging around, so you need to make advertisement focus on point. Facebook groups that are based on crypto?
Specifically how to get free bitcoin? Works. Twitter people who have a lot of followers regarding crypto? Works. Instagram pages about crypto that has a lot of followers? Works. As long as you find your audience, it doesn't matter where they are, you can check the weirdest places and as long as it works, it works. Hence you should always look at who you are advertising to, not where you are advertising at.