I take it seriously, because there's no reason not to. It's a medium for interacting with people. The fact that it's mostly through text, and they're not in my immediate vicinity, should have little bearing on the matter, because ultimately it's real interaction with real people.

But that's not to say I'll freak out if I get insulted online. Not because it's less of an issue for being online, but because I don't care if I get insulted IRL most of the time, so it doesn't bother me either way.