500px.com seems a good site but I'm a little suspicious of all the sudden media attention its garnered in the past three weeks. Mentions in the tech news, an interview with the founder on one of the popular weekly podcasts, mentions on three or four other influential social media venues, mentions here and in the DPR forums. All of a sudden as if out of nowhere a few of the celebrity photographers in social media all switching in there out of flickr.
What's driving it?
They just started seeking investor money, and nabbed their first fish with a sizable sum. If they or the investor are driving bux into marketing the site to build it into a bigger business, they're doing a good job.
I don't see anything wrong with flickr for what I use it for, but that's not marketing my photography. 500px.com looks like it might become a good place to start marketing your photography, if that's what you want to do.