Reddit has a post detailing how their site works and speaking about their success:
“Over the past 15 months, reddit has tripled in size. Since last May, we’ve grown from 7 million monthly unique visitors to 21.5 million. Our pageviews have exploded 4x to a staggering 1.6 billion pages served per month.
“This growth brings new diversity, new opportunities, and new challenges to our communities. There are now over 6,500 subreddits with over 100 subscribers. As we welcome new members into our communities, I’d like to take this opportunity to clarify how reddit works and what role moderators and admins play in the process.
“The most important fact is that reddit is not a single community; it’s an engine for creating communities.
The last sentence is what’s fundamental to Reddit’s success. They’ve managed to succeed despite awful uptime/reliability, financial limitations, serious turnover, and difficulty selling ad space. The fact that new subreddits are popping up all the time allows the site to fulfill niches even as some subreddits become too popular to be particularly useful. It’s happened before (Digg, Slashdot, and slowly Hacker News), but with Reddit, it can only happen locally–within a subreddit. In the subreddits I’ve followed, I’ve seen the deterioration as Reddit has gotten more popular. The CS reddit now has a ton of high school students or undergraduates asking for career advice (Should I go to graduate school? What books should I read to learn about [thing that this exact question has been asked about 50 times]?, etc.)
I still don’t think they’ve solved the problem of deterioration. Would someone start a new CS reddit because the old one kinda sucks? Well, yes, but though there are many attempts, they don’t seem to take hold. However, this formula of letting people build their own communities rather keeping things monolithic obviously has promise, and has kept the site from deteriorating as quickly as it could.