I think it's better than that. A Tumblr blog isn't just a journal or diary; it's more of what was known as a commonplace book.
Commonplace books aren't just for recording your thoughts and personal histories. They include quotes, stories, drawings, ephemera, and other pieces of things that catch your attention or that you find significant. If one heard a poem they wanted to keep, or saw a field they wanted to sketch, or had an insightful thought about a play, they would collect all that in the commonplace book.
Isn't that what we do here? We collect and create art, and jokes, and commentary, and political thought, and a lot of us cram it all onto the same blog and throw it at our followers who can then choose to keep it for themselves or let it pass by. We're basically in a salon passing our commonplace books to one another and occasionally copying a passage (reblogging), making an addition (reblogging with commentary), or making an aside to our companions (commenting in the tags/replying).