The original was 1944x2592, and weighs in at just over 1MB. The version above, which Dropbox magically transfers to my laptop/desktop, I then resized to half of those dimensions, (972x1296) and 248k. It's displaying at half that size again, but you can click to show it at it's actual size.
Hitting the "Upload to Facebook" button produced a version that's 720x960, and 70k. And, unsurprisingly, very pixellated.
Uploading it to Tumblr gave a file that's 960x1280, and 190k. The auto-reposting by IFTTT to Twitter didn't change it any further. The auto-reposting to my Facebook links page again resized it to 720x960, so I assume that's what FB automatically resizes to.
Original post on Dreamwidth - there are comments there.