Doesn't matter. They don't create the content, but merely facilitate creation by unpaid volunteers.
Yes, they use ads, and if they didn't, I probably would be happy to pay or donate (if only to keep all the topical forums in one place and not revert to dark ages of phpBB and linear discussion threads). But let's not cheat ourselves - the ads are not paying for the content, they are merely supporting an efficient platform.
I like reddit because they encourage people to visit the websites of the content creators by way of link aggregation. You go to /r/Worldnews or something, and it's a link to the BBC or Reuters. They don't host the articles themselves. This sends much needed traffic to the people who deserve it. Whether you're a big name like cnn.com or a small publisher like StevesBlog.com, you're on an equal playing field.
Yes, they use ads, and if they didn't, I probably would be happy to pay or donate (if only to keep all the topical forums in one place and not revert to dark ages of phpBB and linear discussion threads). But let's not cheat ourselves - the ads are not paying for the content, they are merely supporting an efficient platform.