OP, I have to ask: How much advertising have you seen on social media for straight-up discussion forums like bitcointalk? I don't use social media, so it's an honest question--but I would be willing to bet there isn't a hell of a lot. Those other services you mentioned (Stake, Sportbet.io) have a huge incentive to gain new members, as they're trying to sell people something.
Yeah, these are some of the very reasons why it's entirely unnecessary to have the forum take out any advertising packages gor itself. It is a free to air platform where it gets almost nothing from the lots of users that gets registered and logs on the forum.
Also, bitcointalk operates very differently from your typical social media where you've got incentives on all that is done, writing a message, posting pictures, videos and it's all about connection. Connecting knowns and even try to relate closed located persons.
The forum is all about teaching although, there are other financial transactions that could be done here but mostly, its more about getting to educate users on the cryptocurrency innovation. So far, it's been doing great and the numbers aren't lacking.
This is why you've got, betting platforms, new projects, mixers, exchanges and the rest of them finding there way to the forum due to the numbers here.
The forum and bitcoin are one in itself and anywhere you see a shiny B with two strokes || (
BTC) between, its indirectly advertising bitcoin for free and a search on bitcoin, the forum wouldn't be far behind.