In reply to Blue Straggler:
I use Flickr, and am a member of quite a few groups there. Whilst some are good for sharing images, and finding images of places and things I'm interested in and locations to take good shots, I've not found any that are good for useful critique. That doesn't mean they don't exist of course; it may be that the group discussion threads are the place to find this, and they aren't something that have ever engaged me a lot.
So I'd say look at Flickr groups by all means, but have a look at the discussions within the groups particularly. Comments on photos can range from useful to a gushing 'hey, this is just fabulous' and though that's nice to know, it's not ultimately that helpful if you're after informed, objective and helpful comments.
Just my £0.02, of course. Others may have a different experience.
T.