Indeed, in most of the countries, restrictions aren't that with online gambling unlike with landbased ones. Therefore, engaging with social media advertisements would still benefit a gambling platform. Also, they won't spend money for something they do not know about so more likely those gambling sites who does so, have tested and saw the results of ads 'coz they'd be throwing their money if they won't know the negative consequences of it. Also, advertisement in any field doen't need to be too detailed. Recognition from players and potential players would be an enough result of social media engagement thru ads.
If I remember correctly, I think I read somewhere that Australia is about to ban online gambling ads from the country, and gambling platforms can't advertise their services online. Now, I'm not really sure if they are also going to ban online gambling platforms or if they are just banning the ads for the concern that it may lead to luring the younger generation towards gambling since we all know how enticed the young generation is with the digital world.
However, I'm not sure if that is the case with other countries and what their take regarding online gambling and their advertisements, but I feel most of them only allow casinos operating online with a valid license acquired from the licensing firms and authorities of the same country they are operating in.