Every week I send out "The Monday Picture" by email and before I joined 500px, I promoted it using twitter, an iPhone app I created of the same name and uploaded it to Flickr. No more! "Sunrise" achieved 13 views and no comments on Flickr but when I uploaded it to 500px 61 people viewed it, two people voted for it and I had one comment. Not a runaway success but a far better result that with the alternative more well established photo sharing site.
If this doesn't convince you to join 500px here is another example...
I uploaded "Ravello" two weeks ago and within minutes comments and votes were coming in fast. Within about an hour it had a rating of 93.3, this has since fallen back to 76.5 but in the fourteen days it has been on 500px 529 people have viewed it, 33 people have been kind enough to vote for it and 21 people have added it to their favourites list.
There are some great photographers uploading thousands of images to 500px every day and the team behind it are working to make it better all the time. It may not be as big as that other one but I don't think it will be long before 500px is the No. 1 choice for photo sharing.