I think Tumblr is kind of gif central, which is like Twitter and Flickr combined only more gifs, kinda sorta.
Tumblrs gone through some changes like Twitter and Flickr have changed layout and added features over the years but, years ago Tumbler was sort of the place to be if you were into micro blogging and gif collecting / making. It was like Facebook for graphic designers, maby? Its hard to explain but eventually more and more people on Tumblr came over to Twitter as it gained momentum.
I never had a Tumblr page and I didn't really learn much about it till a couple years ago, I had friends that used it but they never really talked about it and I was pretty much wrapped up in the audio world side of things. But since more and more people started double posting their gif/blogs to both Tumblr and Twitter I started discovering some really gifted artists like http://kidmograph.tumblr.com/ and http://maxcapacity.tumblr.com/