> voters are not individual agents but rather a mob subject to manipulation by propaganda?

That is true, yet it's not incompatible with democracy. In the US Horace Mann established the foundational link between education and democracy. It's why civics and other forms of intellectual self-defence are essential.

The problem with social media (and BigTech lazy "convenient" non-thought) is not that it's a propaganda conduit as much as that it's antithetical to critical thinking. It's more complex than simply the content, it's the form too.