Flickr wins out (despite that awful UI overhaul) and will continue to dominate partly because of its established user base but mainly because no one offers anything to rival it.
Google Photos offers less than Microsoft Onedrive, especially if you have an Office subscription.
Like most of the recent announcements at I/O, Photos appears attractive at first but closer examination reveals it to be for casual / personal use only.
Flickr has a huge user base because it was purchased by Yahoo. It's probably the most popular site for storing photo's but it certainly has worthy rivals. Flickr is great if you don't mind the ads and simply need a place to dump photos
en masse but beyond that, it's pretty much a wasteland, IMO.
I think we're going to see a real rise in the popularity of Google Photos, or Google + or whatever it's called, if for no other reason than the huge popularity of Google. The Google/Android connection won't hurt either... I see Google Photos being potentially huge.
500 px is a personal favorite but 500 px is more about uploading your best work and networking with other people passionate about photography. For that reason it's garnered a bit of an elitist connotation, and I think they're probably fine with that. The community is certainly harsher than what you'll find on Flickr but I like its clean user interface, its "Pulse" rating system and how I can have my own profile.
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