As photographers rail against sites like Facebook, Flickr and now Google+ for their heavy handed Terms of Service (TOS) regarding how they can use your photograph's, 500px seems to be gaining momentum as a photographer's favorite.
Taken from their About Page:
Should you join?If you're a photographer, 500px is your connection to a whole world of people. It's the place for you to upload your work and create a beautiful portfolio, join in the conversations, vote on photos and get reviewed in kind. We promote all the best works through a variety of social media bringing you the coverage you need to move forward! If you're not a photographer, you should still join! You’d be able to follow the works of your favourite photographers, create lists of photos that amaze you, express your opinion in comments and by votes, and even find a photographer for hire. There are many possibilities and we think that you’ll enjoy exploring them. See them for yourself.
All things considered, most people uploading images to Facebook, Flickr and soon to be opened Google+ don't worry much about a TOS agreement. But if you have an image you have hopes of selling and want a site to display and attempt to sell through, 500px might be worth looking into.