A large amount of twitter employees left or were fired to where to some concerns over how well Twitter can sustain itself. Even if it doesn't break down there's suspect that a large amount of issues can appear.
Tumblr to my knowledge had some problems after efforts to stop NSFW that improved as they added guideline markers. So far Tumblr is much more lively to my surprise despite effectively starting at 0 (Nuked an old account I had as a teenager, most following accounts have long since left the site).