I have had a free 500px account for a couple of years. So far I have only put up 36 photos. I don’t really do much with them. It seemed like you had to be social to advance photo scores and I can’t be bothered. Other sites have more interesting community, though at those sites too, there is occasional discontent that someone has not had their photos featured. This has lead to discussions about limiting upload allowances and even one or two people leaving for greener pastures. When I joined one site, I had a couple of photos featured that I though were fairly pedestrian. It turned out the site’s creator was trying to encourage us. I think they are still doing that.
The best way to determine how others see your photos is to enter lots of contests. If you can regularly win or place then you are probably doing something right.
To be fair, there are lots of good photos on 500px. PBase is another site with good photos, but they have a one month trial, then want you to pay. Since I have my own pages, and they wanted US$, I passed.
500px are local. I just called their posted phone number and was forwarded to a cell phone which offered to record voicemail. I don’t know if they are growing or had a major exodus. They have a lot of positions open.