How I believe Tumblr has helped my career: The Burning House allowed me to quit my job working as a concept designer in New York and travel around in a VW van. This experience has changed my life. It's been a source of inspiration for my photography and websites. The success of these subsequent online projects has set me up with consulting and photography work.
My advice for artists on Tumblr: If you cant explain the concept in a minute, people probably won't be able to understand it. Peoples' attention spans are short, especially for blogs.