There is a lot of discussion among professional photographers about both how bad Flickr has become and what great opportunities 500px gives to photographers. I have to admit that I share most of the views on these 2 services. I thought at first when 500px entered the market that would be more complementary to Flickr, however I quickly realized that 500px is actually filling the gap and opportunities that Flickr has missed long time ago.
I'm slowly abandoning Flickr and although I have a pro account at the moment, I'm having serious doubts whether or not I'll renew it in April next year. Flickr isn't a social network for photography lovers anymore and it's not even a valuable photo-sharing service anymore for me. The possibilities for valuable interactions are minimal when compared to 500px and most of the comments on the photos are just plain spam (great work, join the group blablabla or something like that - hate that!).
Although I believe that there is still lots of great work posted each day on Flickr, I find the service horribly polluted with regular pictures and it's really hard to find those good photographs, to learn from others. Not to mention, getting valuable and constructive criticism on your work is almost impossible. I have to admit here that I'm not as active in posting comments on Flickr as I used to be. There are several reasons but I think that the main one is that 500px (not to mention Google Plus which is HUGE among photographers but it requires a different blog article) is much more valuable and reacher for photography enthusiasts. I prefer to explore photographs on 500px and spend my time there rather than on Flickr and the reason is simple: 500px enforces you to upload only the best shots - that's how the community and the system (for free accounts) are made. The work there is stunning, the comments you give and receive are truly valuable (at least most of them) and you can easily get constructive criticism on your work and learn from it.
It's really up to you and your preferences if you want to use any of these services and which one to choose. In fact, you can choose more than one but it's also the time issue for me - being on too many photography services is a bit of a waste of time for me. I just prefer value over quantity :)