Is Tumblr shutting down?

No, not true. Someone else posted that. It's just an attempt to get as much reblogs as possible. Kinda like those chain messages in e-mails. The ones that go "send this message to your friends or else barney will kill your kitty". Keep in mind that you can edit the source in the post editor. You shouldn't believe these stuff unless the original post was from the staff itself.