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.
The thing is that gambling promotions are prohibited in some countries where gambling is term as illegal such as some Islamic countries where gambling is labelled as haram, so for that social media within that country will block all gambling-related adverts.
But if a citizen of such a country who already knows how to manipulate his way into bypassing such restrictions, the promotion can still be pushed through with such promotion on social media, In my country some social media platforms block casino adverts.