Dear Ruby,
How do you deal with people in the world who project their rampant unhappiness onto you?
I'd like to know, mmkay?
Sometimes it's best to just avoid them. Sometimes you can't easily avoid them, because they're your boss, your co-workers, or are a member of a forum that you enjoy otherwise besides that person. In that case it's best to limit the damage they do to you as much as possible.
Try not to give them any ammo to use against you. Self-deprecating humor, I've learned can easily be turned into something malicious. I have made jokes before about something about myself in a chat room I went to and had them turned back on me even though they were supposed to be light hearted. It was really ridiculous, especially when I was just trying to build some camaraderie and didn't realize how vicious people were. Mild silliness can be turned back on you to make you look stupid, etc. so if someone is not a nice person you know you can't really have too much fun with them without being on guard.
Remember that if you know you haven't done anything to merit a nasty response from the person, then it's their problem and not yours. Try not to take things personally even if it's directed at you. That's one of the things I had to learn as a cashier, because sometimes customers would have really rude, nasty attitudes toward me, but I had to realize they didn't even know me and their nasty attitude really wasn't about me, it was about their own lack of manners and whatever stresses they had in their own lives.
Sometimes it has helped me in the past to imagine a shield around me, and the bad energy bouncing back off of it. It also helps to pray for protection.
On a forum you can often just ignore someone and that works fairly well, especially if someone is just baiting you for "lulz".
Unmoderated Usenet Newsgroups are of course the number one place you have to have thick skin in, chat rooms being a close second, and forums (being more moderated), are a little better.