I've been using Scribophile for a while and unlike other writing sites this one is focused on getting feedback on your writing. If you want to just post your writing for the world to read, like in a blog, then this is the wrong place... it's for getting reviews and critiques. And they're really good quality. It's a little hard to earn karma points but for me writing critiques has been a good way to train myself to look at my own writing differently. Great site!
We're by far the largest, but that's not always the best thing. Some of the best small groups on allpoetry get full sometimes and aren't accepting new members. For me the best kind of feedback comes from a dedicated group where the same people review each others poems each week - you can find that at a local poetry group, or in some of the small groups on allpoetry. There's also a "Trade comments" feature but it's not as good as making good long-term connections...