There is also the 'pyramid' effect you can get by owning signups. If I as a content provider or just a referrer send you in as a paying customer I can get a cut of your monthly without even serving you content.
And there is also well done advertising. Once you have a big group of people that reads articles on specific topics, I bet companies will want to pay a lot to send them (top quality of course) content that is also promotional in some way.
