[T]hanks to the fabulous Tumblr Crosspostr plugin developed by Meitar Moscovitz, we’re still publishing In Brief to Tumblr, so as not to leave any of our Tumblr followers behind! In Brief on Tumblr is now basically just a mirror that will automatically update whenever we publish a new item with WordPress.
This migration was in the works for a long time. We began planning to leave Tumblr a few years ago when it became evident that Tumblr wasn’t innovating to the extent we had hoped it would when we first began using it in 2009.
Tumblr’s glitchy post editor became a hindrance to seamless publishing, and Tumblr’s decision to abandon its BlackBerry mobile application and discriminate against BlackBerry users bothered us too. Tumblr never made an effort to address our grievances… so we left. We’re much happier on WordPress, which is free software that’s totally under our control.
Emphasis added. I can not overemphasize how important it is to own your own content.