It should be mentioned in these types of lists. It's not fair to say "Facebook, Twitter, Instagram, Pinterest, and Snapchat... and the rest". Reddit is huge
Not just in DAU either. Mind-share wise, it has a huge influence on the wider web and is increasingly the platform for content stealers (the 9gags, buzzfeeds, and facebook pages).
9gag has been caught time and time again using bots to just rip off whatever was trending on reddit's r/all feed (most obvious when they copy SubRedditSimulator, which is just a Markov Chain recombination of other trending posts).
Is there an open source forum-CMS like phpBB that works like imgur but can be self hosted? Discourse would be the only candidate I can think of and I don't think I could teach my welder buddy to admin it for his welding group (he doesn't like FB).