I like 500px a lot, the interface is great. I don't just use these sites as a place to post my works, I use it to browse others' works, and Flickr Explore's standards can be seriously bad sometimes. 500px has its fair share of bad work under "Popular", but "Editors' Choice" is quite consistent (albeit slow to update)... The common trait to both is that XMM/scantily clad model photos are very popular. Guess you can't really stop the testosterone from flowing all over the place..