Twitter is well funded and big enough that there would be signs of collapse before it started to die. YouTube is owned by Google and has tons of advertising. I'm not worried about either of them.
Reddit is owned by a publishing company that's lost interest in keeping it around, it's not nearly as big as Twitter or YouTube yet, and it has one of the worst audiences imaginable for monetization. I wouldn't say it's dying per se, but they have to start thinking about how they're going to pay for all those new servers and all that new staff.