I don't try to get feedback on every page. I figure I've got to have some faith in what I'm doing, and that would bog the whole process down a huge amount anyhow, especially since that would pretty much require telling people the story ahead of time (I'd be more likely to do this if I was drawing a humour strip).
If there's something I'm particularly unsure about and I don't have time to stubbornly research everything possible about it myself, I usually ask my family. They do tend to give me useful criticism, and most of them are artists or writers in some capacity, so that's better than trying to ask people it might take a while to get replies from. That said, family also tend to not be the most unbiased of people, so I also like to get some feedback from other people when I can.
I know this definitely isn't true for everyone, but for whatever reason, I've generally had good luck getting useful feedback on DeviantArt. Some of the most useful constructive criticism I've ever got has come from people on there. I don't know so much about other community art sites, but it seems like the same would be true of most if not all of them.