Not all social media platforms would be an effective way to advertise any casino site since not all of their users arent into gambling. I would prefer gambling-related platforms and forums which discuss things about gambling that also have a huge number of target players. A good advertising strategy has a huge positive impact on the site and could also help in building up a good reputation.
Both have their good and bad points, advertising in websites which are specialized and talk about different gambling topics are a good option to advertise a casino, but most likely all the users there already have a favorite casino so it is going to be difficult to convince them to try another one, and while using a social media campaign means that your advertising now lacks a specific target audience and that many people are not going to be interested in your casino, the few that are interested most likely are new gamblers, which will be eager to try your casino right away.
You have a point. But even gamblers already have their own preferred casino for playing, there's still room for new casinos to became well-known. Its just depends on how they will market their casino, like their unique features and generous bonus that can attract the gamblers. Regardless if you promote in sites or social media platforms, the chances to succeed depends on the strategy to gain the attention of the gamblers.