Study suggests people post what they expect their followers to agree with – in other words, Facebook becomes an echo chamber that stifles discussion and debate. Few people will challenge their “friends” when there are different interpretations or even when the facts are completely wrong.
A side effect is that people become dumber over time as they learn things that are not actually true but which go unquestioned. Propagandists know the first message delivered often sticks, in spite of later contradictory information. As a consequence, social media has become a frictionless conduit for the unimpeded flow of propaganda messaging.
A related issue is that as elections approach (and for some people this seems to be a 12 month long event, held annually), much political propaganda is posted online. And much political propagandizing falls into the category of online bullying: anyone who does not agree is obviously wrong or stupid.