I have no idea how this has happened, but I have not had my art stolen. ... or I have, but no one's ever told me about it.
Maybe it's because I don't draw a ton of fanart. I've drawn more of it this year than any year before, and it STILL isn't all that much - and people always seem more prone to stealing fanart than original art, for obvious reasons; fanart is more popuilar.
If someone stole my art and reposted it on, say, Tumblr and claimed it was theirs, I'd be disappointed and try to get them to take it down, but it's more annoying than anything else. I slap my signature on most things that aren't comic-pages, and if they go through the trouble of removing that, well, whatever. I'd definitely be more upset if someone was selling my artwork without permission, because that's very literally a loss of income for me.
But otherwise...? Meh. I suppose it's the fact that I know I can draw more things just as good, and these people are stealing because they either can't hack it as an artist, or are too impatient to just learn how to draw on their own - I can make more things, and they can, at best, do empty imitations.