The biggest is that they have, quite intentionally, encouraged the spread of outrage and mob mentality all across the political spectrum. These companies have installed levers for manipulating their users emotionally, and these levers are for sale. You can even fiddle around for free if you have the time, or get (un)lucky!
Oppressive PC culture, that can manifest as an almost Puritanical form of moral authority, spreads quick on social media. As someone who grew up in conservative, small-town America (and rebelled against it every chance I got), I'm not about to have my opinions dictated to me by the new boss either.
If your social sphere is mostly composed of middle & upper class, highly educated professionals, you might just feel surrounded by PCness. If you have any connection to the white poor and/or rural however, you also know that right-wing authoritarianism and racism are boiling too, with lethal results. As far as PC culture can be viewed as a defensive response to racism, I think it is clearly the lesser of the two evils, but it's probably a false dichotomy.