For example, Anonymity plays an outsized role in the equation. FB is as you said essentially the same as Reddit except (most) people self censor to a degree that keeps it mostly cordial. They’re not going to use certain language and allow their full opinions to be heard, because someone they actually know could be reading. (Some people don’t care and post unfiltered but speaking of the majority).
Generally People posting on LinkedIn is often going to be some form of HR speak, they don’t want to offend anyone in their professional network. They’re not going to talk about religion and politics and whatnot.