Well just look at the existing propaganda machines online and how annoyingly effective they are
And about your thought: I disagree. When I look at those online places, I see echo chambers, trolls and a lack of critical thinking (on how to properly discuss a topic). Some parts might be artificially accelerated, but I don't see propaganda couldn't be fought. People are just coasting, lazy, group thinking, being entertained and angry.
Propaganda is less about what it says, but more about how it makes people _feel_.
If you get the feels strong enough, it doesn't matter what you say. The game is over before you start.