I've noticed a lot of tumblr blogs I used to enjoy simply stopped being maintained by their creators. In some cases they've even been deleted.

There still seems to be a vibrant community on tumblr, but it's different from the one that flourished five or six years ago. A lot of the design-focused bloggers have simply left.

AFAIK, Tumblr closes blogs that receive too many copyright infringement notices. That's probably the reason.