First, I'd like to point out that there are a ton of things you are legally not allowed to say. The example everyone is familiar with is that you can't yell "Fire!" in a crowded theater (everyone will kick your ass for talking during Rogue One). But there are many more examples of things you have no legal right to spout off. You can't incite people to violence, you can't slander (in speech) or libel (in writing) someone, and you can't say things that would make any reasonable person punch you in the face, because them's fightin' words (though telling someone you thought The Force Awakens was a good movie is still technically legal, for some reason).
Yet despite it being completely illegal in real life, people think they are allowed to threaten and harass people online. Leslie Jones received a mind-boggling number of inflammatory and threatening messages on Twitter, and zero people went to jail. Contrariwise, if someone (say, I don't know, maybe a Cracked writer) organized people to make a bunch of prank phone calls to a radio DJ, they would for sure go to jail. Just ask Cracked writer and jail alum John Cheese.
So just to be clear (and I can't believe this is a sentence that actually needs to be written), you aren't allowed to intentionally inflict harm on someone, even by just using words, whether via in-person chat, phone, email, Facebook, Instagram, telegraph, Snapchat, Tinder, smoke signals, singing telegram, carrier pigeon, words scrawled on a gas station bathroom wall, or even Reddit.
Or whatever 12 new garbage apps got invented while you read that sentence.