Toxicity isn't the only factor. USENET[0] may have been nasty but that nastiness was also very public and untargeted. Social media adds the ability to be extremely targeted with your abuse in ways that are nearly invisible to those who aren't clued into it. You can weaponize it way easier than you can a message board.

[0] And, for the purpose of this discussion, imageboards and other forums with little moderation, such as 2channel, Futaba, 4chan, etc.