I post mine on my own site and also Tapastic, The Duck (aka Drunk Duck : P), Smack Jeeves, and Comicfury. And I post preview images on Facebook, Twitter, Google Plus, Tumblr, and Instagram. It's a bit of work, but I'm kind of addicted to the feedback I get collectively from all these places, since my own site doesn't have page comments.
Also, last I checked anyway, which was years ago, I did get some traffic from the mirror sites to my main site, so it's more or less free advertising; and the social media sites bring even significantly more traffic than that--especially Instagram, once I discovered only recently how to use hashtags there :P. Instagram alone--and I'm not at all a popular account on Instagram or anything--took my main site from something like 3-4k page views / day to 6-7k/day. : o