See, Twitter decided early on that it wasn't going to pander to its original base. Smartly, Twitter decided to ignore the demands of edge-case geek users like Robert Scoble, and instead focus on mass appeal, celebrities, and building out the core platform. The robust and easy-to-use Twitter API has spurred a flourishing ecosystem of third-party apps.
Tumblr's doing the opposite. Rather than focusing on expanding its audience and making it into a valuable platform, it's coiling in on itself, doubling down on the un-monetizable memes that its core users love -- like pictures of sharks and cats. A non-Tumblr user joining the site would have no idea what the fascination with cats and sharks is all about. It's totally self-referential.