I was getting excited about using 500px for the specific reason that it seems to load much faster than flickr. Then I loaded it up in a public space to get some inspiration by browsing other user's work - interspersed were various images of women in various states of undress. I quickly exited the site and never went back for fear of someone glancing over my shoulder and assuming I'm doing something other than getting artistic inspiration.
Not arguing that the images aren't art but I personally don't want them popping up on my screen when I'm just trying to get inspiration for landscape photography.