No-one is immune to propaganda.So basically you're saying that as long as you insulate a moron from harmful beliefs, they'll end up benign? Keep Lennie away from the rabbits? There's something to be said for that theory. Stupidity can be seen as a function of how easy it is to trick someone, so if there are no tricks there can be no stupid. Or something to that effect.
On the other hand, humans have kind of an immune system when it comes to manipulation. If someone goes their entire lives without being exposed to evil, they'll be unable to identify it. Most genocides were committed by very stupid people who spent their entire lives as unthinking cogs, and they continued to act as unthinking cogs when someone gave them guns and told them to kill. So it might actually be beneficial to let stupid people fester in their stupidity for a while and pull them out before the point of no return. It's hard to say.
Propaganda is a highly researched field that is constantly evolving with the intent of deceiving people. It's easy to look back on past atrocities and say that people were stupid, but it's easy to look back on many scientific beliefs and say the same thing. People used to believe that the Solar System revolved around the Earth, that disease was spread via foul smelling air (Miasma), that bleeding actually helped people and countless other stupid ideas that were abundant. (Lobotomies are my favourite)
Facebook has offered targeted ads towards people who are liable to believe ridiculous claims, all in the name of the Greater Good that is unregulated capitalism such as allowing advertisers to target "pseudoscience" believers. You can bet your ass that Zuckerbook is tracking and advertising certain groups towards people that fall into certain categories, many of which are either creating propaganda or repeating propaganda of other groups.
The common phrase "What doesn't kill you makes you stronger" is bullshit, and exposure to propaganda has the same effect.