Our "The Depths" webcomic:is scattered clear across the porn art sites. I have absolutely no way to get them taken down, because it would mean registering and speaking to web hosts and maintenance that will likely not give me the time of day to do anything about it anyway. A lot of them are in different countries too, which means I really can't do anything there either.
But what we found we COULD do? We interestingly enough have furry acquaintances all around the world who have access to these sites...Sooooo... we asked them to kindly make a comment where they see our pages and spread out the link to the actual webcomic site.
And man, did our traffic explode. 
So, if you find your work somewhere you don't want it, and the webmasters will not help, find a willing participant to sign in, and add a link in comments. It really helped us. And comments normally don't get deleted. 
