I think the main problem with 500px is its operational obscurity. That and the fact that the scores seem to be driven by a certain amount of sycophancy.
There is, for example, no 'help' link, no getting started guide, other than a rather glib and shallow tour. I'll persevere with it from time to time and see if it clicks with me.
I have had at look at ipernity, it does seem rather superfluous. And I really don't have any significant issues with Flickr. Although it seems that Lightroom is having one of its funny five minutes and won't upload anything to it at the moment. Did I mention I had a wilful router?
Must get a new one.