Worse than just believing it … these days they can (nearly) instantaneously share it to zillions of other people around the world many of whom will also believe (and re-share) it, and few in that chain will bother to fact-check it themselves, and of those few, even fewer will take the time to call out the error / misinformation, and even if they do, they'll almost instantly be shouted down and ridiculed by all those who just outright refuse to see facts when they really really want to believe a lie. This of course leads the average "volunteer" fact-checker to just sooner or later give up even trying to fight the growing waves of "fake news", and keep their fact-checking to themselves; Not good for anyone.