I think that gambling advertising on social media is ineffective and very expensive, since gambling is basically prohibited in most social media
From my own personal experience in working in marketing, I would have to respectfully disagree with you. Any talented promoter or social media growth hacker worth his salt would know how to navigate the bans on gambling advertising and create a decent marketing campaign using social media.
Every medium of communication is very important as far as marketing and advertising is concerned. A smart marketer would always look for ways to do their work so as to pass across their information and communicate with the masses they target to have in engagement. So your stand in disagreeing with him is not bad as all social medium platform are open to advertising because they generate funds and revenue from it as the traffic increases to the site due to the advertising.