I’m not sure if this comes under social networking sites really, but it’s been a while since I did the first one. On Brawl Snapshots, a Game Cults site, users upload stages and photos of Super Smash Bros. Brawl and if you like them, you +1 them.
Good photos are featured and there’s contests too. You can also download any stages you see on there to use on your Wii, which is wicked cool.
My profile & gallery are here and I’ve made a few stages like the Pixl (allies from the game Super Paper Mario) ones, which I particularly like. Thoreau is my favourite design and one to battle on, so please let me know what you think of that.