Tumblr launched in 2007, during the blog boom. Folks believed the thing you did online was write and talk about yourself. It doesn't have to be interesting, or insightful, it could just be a little diary about your life that others could see if they wanted to.
This is why in 2006 when Twitter launched, people were amused by it. What am I supposed to do with 140 characters? Talk about my lunch?