I post the rare piece on dA because I know I can ninja it out of existence if need be, and that's really it. I tend to write poetry with publishing in mind, so I don't put it anywhere unless I can set fire to every trace of it having been on the internet.
Nowhere. Mostly I write novels, which I am interested in having published eventually and which I don't think read well online (never mind getting committed readers). Posting online is generally not feasible for my goals, so I tend not to. Unless I decide to write something short for a contest or am not interested in publishing. Or haven't been able to publish for whatever reason.
I use FictionPress.com or Fanfiction.net. It seems like you're more likely to get more useful feedback on those sites, whereas here it's more along the lines of "zomg so kawaii! " Not that that's a bad thing, but it's not exactly helpful.