I have had most of them....and the one I eventually came back to? Flickr.
500px is essentially a beauty contest. Think of those "Am I Hot or Not" Sites. That is 500px.
My Website never got any hits and was a waste of money. In the 5 years I had a site, I maybe got 100,000 hits, and that was mostly from bots and little boys who wanted to see the half naked picture of the chick with big boobs.
Facebook is for my friends and I seldom put anything up on that any more. Even when I had a separate page for my photography, no one seemed to give a rat's ass. So I closed it.
Instagram I am still using for cute photos of my doggie. Random people actually follow me, but no one gives a rat's ass either. If it wasn't for the astrophotography and the half naked women, I would probably drop it too.
And Flickr. I upload monthly for my friends and family to see what I have shot. It is free and occasionally people have something nice to say.