Have you ever been stalked by someone you didn't even know online?

I have been several times,on a friendster-like website where i went to talk to my former classmates and old friends. Some people just do it put of spite,two seconds ago you didn't even know they existed and now-bam!-you're getting hate messages every 10 minutes. I kinda understand it if those people were somehow connected to someone you know and be jealous or something but the problem is there's no freaking way they could even know you:)
Why do you think those haters hate? My explanation is that makes them feel more important and noticed or they're just so full of negative emotions that they just go crazy. Also,with this modern life,I can't believe some people think that it's not enough to have real enemies in real life so they go and make new ones online:)
How do you act in this sort of situation? I usually just ignore them