Well, mostly, they replaced Usenet and a hodge-podge jumble of forum sites running generic interfaces where you have to click to a new page for every 50 comment

Well, mostly, they replaced Usenet and a hodge-podge jumble of forum sites running generic interfaces where you have to click to a new page for every 50 comments (or less), which have no comment reply nesting. Reddit brought a more useful/readable interface, allowed anonymous accounts with no email address, and had a very simple & powerful (if flawed) voting system. This attracted content creators of all kinds, which led to a rapidly growing audience, and it snowballed. I don't like one company owning all this stuff, but I do love what users in niche subs have created there. With a massive general audience comes better populated niches, excellence riding the coattails of the mediocre. Also, should Condé Nast ever screw the pooch here, plenty of clones are waiting in the wings.