They also have, as I understand it, a metric fuckton of content sources (not including Flickr, Tumblr and so on) and a very highly-tuned content and advertising

They also have, as I understand it, a metric fuckton of content sources (not including Flickr, Tumblr and so on) and a very highly-tuned content and advertising presentation system, plus the default footprint on various browsers. Essentially, they have a lot of default eyeballs and use that for advertising. Unfortunately, they've relied on rent-seeking in that space, and Apple and Facebook's recent moves may bite directly into their one profit center.