Obviously Facebook/Instagram/Twitter/Snapchat kind of worry them, but then again, I feel like people are "done" with "social media" so I wouldn't be _really_ worried if I were Reddit. They have the advantage that they can give the user very interest-focused information; when you follow a friend on Facebook, you get their programming advice, baby pictures, and political rants. With Reddit, you get to skip the baby pictures and political rants.
Also, unlike Digg, Reddit hasn't been bleeding members to another site for some time now, so there's nowhere for Reddit users to go if they become dissatisfied. Does anyone have any ideas about why that is? Reddit is not, fundamentally, a hard product to emulate, and yet I know of no site even remotely as popular nowadays (HN aside, of course). Slashdot, Fark, and sites like those all seem to have mostly been consumed by Reddit. Is it the strong network effects that Reddit has, or does Reddit's moderation system provide a unique way of hosting a community that no one has reproduced?
I think Discord has been siphoning off some of the users on the gaming related subreddits, but it fundamentally serves a different purpose so its not a wholesale replacement.