I was under the impression that 500px was for very serious amateurs or professionals and even then it's more about having a blog and portfolio than sharing your latest pics.
What is it you don't get about Flickr?
I've been using my (paid for) Flickr account properly for about a year or more now.
I tend only to add contacts of people whose work I like, I've not seen it becoming like photobucket, sure some people just upload to host their photos somewhere, but I don't see that as a problem and I don't often see those photos.
There's a lot going on on flickr, but I like seeing that people are viewing my photos (70,000+ views right now), getting feedback on my photos and finding out if people like it or not. I also like seeing other people's work, appreciating it and being inspired in my own work.
That said, I've not really tried 500px, so I might have to join and have another photo site to
waste spend time on,
