I think your argument is valid. Social media platforms incentivize seeking virality which is another way of saying the platforms incentivize participants to seek as much attention and approval as is possible within the platform's interaction sandbox. Usually this is called "engagement" but the principle is the same, engagement is equivalent to attention (both positive and negative) and is one the main reasons that politics is becoming polarized. Extreme opinions tend to be more engaging on these platforms and so such opinions get more attention. More generally, all existing social media platforms seem to incentivize the creation of polarizing content. Recent outbursts by Kanye West are good examples of polarizing content that gets a lot of engagement on social media.
I'm not familiar enough with game theory to comment on the other parts but I personally don't think there is a way to incentivize people in online forums to express truthful and honest opinions as long as the interaction sandbox itself is paid for and subsidized by commercial entities.
