One of the things I like about Flickr is the whole notion of serendipitous community. With most of the other photo sharing sites, you don't get strangers self-collecting by shared interests.
Sometimes it's the good and the bad side of Flickr, though, right? Five-element North Korean lenses? There's a group for that. Foreign fire hydrants near cabbages? There's a group for that, too. I think if Google can ever figure out how to mingle Groups with PicasaWeb, they might have something comparable. I like Google+, but it still doesn't foster the same kind of group dynamic Flickr can.
It might also be intent, too. Are you trying to share pictures with friends or pictures with strangers? Sharing pictures with friends there are a lot of good options. Sharing pictures with strangers is a different ball game.