See here for my attempts at using Tumblr. I’ve never fully understood it and still don’t after having had one for the past 3 years, but I’ve noticed that some of my Flickr photos have appeared on there and I’ve been experimenting with posting to it (particularly since it’s a 2-click process from Flickr and you don’t have to sign in to Tumblr).
What puzzles me is that when my Flickr photo is linked on Tumblr, one assumes that constitutes a hit on Flickr. But, Flickr’s shonkiness stats-wise notwithstanding, it appears not. For example, the Japanese temple photo linked above has had 598 “notes” (either “likes” or “reblogs”) on Tumblr at the time of writing, but it’s only had 125 views on Flickr.
I doubt I’ll get an answer here, but feel free to comment if you know anything of the relationship between how images are shared between the two sites and how audience traffic to both is measured.