When I joined Tumblr: I put up the first FIIMT post on May 17, 2011. Previously I had messed around on Tumblr with a blog called "Babies Taking Care of Babies," which was half an inside joke, half the only fun thing going on in my life at that time.
How I believe Tumblr has helped my career: I initially conceived of FIIMT as a zine, but Tumblr helped make it more accessible. I never thought it would lead to anything career-wise, so the fact that it resulted in having a book published has been an unexpected gift. I am simultaneously in awe/terrified by the power of the Internet.
My advice for artists on Tumblr: Worry about generating material that you feel passionate about, not generating an audience. If you are consistently creating stuff that brings you joy, people will find it.