From our announcement page:
Running Tumblr on WordPress will make it easier to share our work across platforms. We can build something once and bring it to both WordPress and Tumblr. We can run Tumblr on the rock-solid infrastructure behind WordPress.com. Tumblr will benefit from the collective effort that goes into the open source WordPress project. And WordPress will benefit from the tools and creativity we invest into Tumblr and contribute back to WordPress.
This won’t be easy. Tumblr hosts over half a billion blogs. We’re talking about one of the largest technical migrations in internet history. Some people think it’s impossible. But we say, “challenge accepted.”
Although I’m not on the team that’s working on this project, I’m excited about what it will mean for the future of Tumblr and WordPress.
➝ Source: automattic.com

