The line between flirting and harassment seems pretty simple: people flirt online if they are interested. It becomes harassment when the other person says that they are not interested, but it doesn’t stop.
I’ve participated in a few social sites, including eHarmony, Second Life, Facebook, and Fluther. Flirting has been experienced on all four. All have stopped with an explanation of my personal interests or when I asked.
I feel safer on a website where there is control over how much personal contact information is given out and when there are controls, including moderators, that can help manage it. There are enough cases about how harassment and restraining orders, at least in the US, are handled in the real world that it is more worrisome scenario.