First because that is a spam and it is not ethical.
Second nobody normal will join on spam posts
While you might think that, spamming is unethical, but can be effective. Spamming on any platform can get you banned, and I wouldn't recommend it though.
However, taking the idea of facebook, you can join groups related to bitcoin/gambling and post once in a while about the gambling site. That way, it isn't spam, and some people might even find a site that they really love.
It works for any social media, but especially facebook due to the groups feature.
Or, like others have said, advertising is a good method. If you run your own website, you can promote the certain gambling site as well.