Part Three of Paddy Johnson's "Brief History of Animated GIF Art" on Artnet discusses GIF projects on Tumblr.
Tumblr probably had more to do with animated GIFs becoming a "thing" (beyond their initial geek/underground appeal) than any other single factor. The platform had a large enough constituency of "creatives" (not just artists, as Johnson points out) to achieve a critical mass of interactions, and from there this gospel spread to the wider world of casual meme sharers.
Johnson:
A Tumblr-based art world, generally speaking, is defined a little more broadly than the art world defines itself. The dashboard removes context the way a Google image search does, so that may have something do with its democratic nature. Articles and lists about artists on Tumblr typically include artists with little to no connection to the art world—mathematicians, animators, computer programmers, etc—as well as artists who work the gallery and museum circuit.
Let's also add advertising art directors, fashionistas, musicians, and anyone else who sought to punch up a page with animated GIFs. Having all these non-self-identified-gallery-circuit-workers in the stew (making legitimate contributions) renders the would-be art historian's job next to impossible for the GIF phenomenon. An article like Johnson's is valuable because she was there, witnessing these sites as they came and went, and while her focus is Art she's not so dogmatic as to exclude other elements of the mix. While I have very little use for David Szakaly's geometric confections and prefer the more subtle manipulations on Stephanie Davidson's Rising Tensions blog, you can't very well cover Tumblr without mentioning Szakaly. I can also mostly not give a damn about GIFs taking a ride on popular movie clips (a la Three Frames) but that's also a factor a critic has to mention. There is a tradition of artists unpacking media clips, and Tumblr abounds with amateur and professional versions of that impulse.
One possible correction to Johnson's history: I believe Tumblr initially had a 500KB size limit for GIFs, not 1MB. I recall people complaining early on about the arbitrariness of Tumblr's GIF handling (some worked, some didn't).