4chan is notoriously nasty. I don't even know why I visit that site, because I usually regret it. Could it be that most people, deep down, are just *******s? That, given anonymity, they'd be no better than the /b/tards? It erodes my faith in humanity, it really does.
Up until very recently, I was addicted to reddit. It's more civilized than 4chan, but people can still be very harsh and hostile. There's also the voting system, which means that you're pretty much constantly judged on what you post. I would obsess over the votes and replies that I got. I took it all way too seriously, and I let the environment there really bring down my self-esteem. Certain replies also sent my anxiety through the roof. I had a very unhealthy relationship with that site, which is why I had to quit.