I use Flickr primarily.
Luke Kowand | Flickr but using Instagram more and more and more. I typically will post the whole image set form a shoot to Flickr, then do daily highlights on instagram when I'm in the mood. Facebook I usually post the odd one here or there, but I've moved most of my photography posting to Instagram and share out.
I used to use 500px a lot too, but too lazy to upload to multiple placed.
I tried a fun one today, uploaded a few photos to imagur, instagram, flickr, facebook. Sat back and watched what happened and how different the responses were:
Flickr - nothing (not surprising as I didn't tag them)
Facebook - moderate response
Instagram - near instant 25 likes, averaging a few likes an hour now with the odd comment.
Imagur - downvoted like crazy and trolled lol --- edit: the trolling is hilarious!!! Sure they are ripping my photos apart looking for anything to rip on, but I'm just sitting here laughing at the stupidity.