Ehh... I disagree somewhat. 500px is another beast entirely, with its own (huge) set of problems. As an avid photographer myself I can't really get behind it.

To summarize a large argument: 500px has an extremely strong groupthink, leading to a very confined set of photos that its community prefers. It is much stronger than any other photo site I've come across, including Flickr.

Its notion of beauty is very firmly in the territory of tourist-kitsch, and the single-mindedness of its community prevents it from catering to, well, any photo enthusiasts who desire something different. Heavily processed long-exposure landscapes, extreme telephotos of animals/birds, and scantily-clad/naked beautiful women - those three topics alone dominate that site.

500px's photos lean towards high production values, it does not lead to either discourse about photography as a craft or deep sharing. Which is to say, it is unlikely to replace existing photography communities, nor is it likely to take the mainstream like Flickr, or Instagram. The community shuns serious photographers outside of its narrow niche, it also shuns casual amateur shutterbugs like mom and dad's point and shoot, and it shuns "artsy" lo-fi photos like Instagram. Breaking that community out of its self-constructed (and very, very small) box I think will be the 500px team's greatest challenge.