https://news.ycombinator.com/dmca.html
Besides that, [1] applies.
Regarding ad-supported websites, they have their copyright policies and you should respect them (if you care about having their rights respected as much as you care having your rights respected, of course). If you choose not to respect them, it's your free choice to do so and bear any possible consequences - as it is my choice when I don't respect copyright laws, which happens frequently enough, but with my full knowledge that I'm actually violating the rights of the original content creators, and also, in that case, breaking some laws...
[1] If a reader comments on my blog, does she license the rights to me?
When a person enters comments on a blog for the purpose of public display, he is probably giving an implied license at least for that display and the incidental copying that goes along with it. If you want to make things clearer, you can add a Creative Commons license to your blog's comment post page and a statement that by posting comments, writers agree to license them under it.