Reblogging, on Tumblr, is so easy that the vast majority of Tumblr sites actually create little or no original content: they just republish content from other people. That's a wonderful thing, for two reasons. Firstly, it takes people who are shy about (or just not very good at) creating their own content, and gives them a great way to express themselves online. (As Arianna Huffington says, "self-expression is the new entertainment".) And secondly, it acts as a natural amplifier for the people who do create original content - the average post on Tumblr gets reblogged nine times, and therefore reaches vastly more people than if it just sat on its original site waiting to be discovered by people visiting it directly.
There's been a bit of chatter over the past few days about the death of the common web and the changing face of the ecosystem we call Internet. It's all about fishing where the fish are. And unless you've built up a following over the last decade or have a bundle of startup cash (although that even isn't a gaurantee these days) maybe it's better to start on a platform that already has a large pond. As ugly as that might be.