I spent years playing the WC3 custom map DotA, encountering all manner of horrible people. I encountered people who used exploits, people who talked immense amounts of trash, people who would leave if you killed them once (at least one in every game), people who would leave because you 'took their lane', people who griefed, people who would steal your items, people who would abuse shared control, people who maphacked, people who used custom programs to remove you from their games (and subsequently would ban you for 'leaving'), racists, homophobes, 11-year-olds off of their Ritalin, people with 300+ ping, people unable to type coherent words, the list goes on. I sometimes wonder if sitting through all of that was worth it for that one good game that came around once every couple of weeks.
But, in all those years, nothing could compare to what happened to me almost a year ago. Totally unrelated to gaming online, I had someone get ahold of of my Facebook password. This person subsequently posted links to porn on all of my friends walls. This could have been so much worse, but luckily Facebook suspended my account before it got too out of hand. The clean up and explaining I had to do took me hours, and I have since all but given up on social networking sites.