Digg let down themselves. I doubt Tumblr is going to do. If nothing else, they have a decent design taste. That alone is enough to survive and multiply.
They do have good design taste, but their technical side is missing quite a bit. Just earlier today, I couldn't figure out how to submit an image by URL (since they automatically resized my original): kept getting some kind of "invalid file" error (I'm not uploading a file!). Scheduled-publication was also broken the 1 time I tried it. Very frustrating.