We plan to monetize via charging R&D firms for access to 'trending research' data - data about which papers are trending in a given area of research. I am a firm believer that the participants in the scientific community, and the world in general, should not pay to share or access research. Participants on Facebook, Twitter, Wordpress, and other sites don't pay to share ideas, and the same should be true of the way science works.

That said it would be unwise at this stage to rule out some kind of freemium option. It is not the ideal option, but when trying to achieve a mission you want to keep as many options open as possible. You don't want to box yourself in in ways that may damage you later. There are also many good freemium services (Github, Dropbox, LinkedIn).

To achieve a revolution in science, you need to build a huge community, and change the behavior of scientists. There are many ways in which a site tries to grow. Requiring users to sign up to download papers is one of those growth channels for us. If it was a big deal amongst our users, we would change it. But in practice it hasn't been an issue; our users don't mind this. Most people read the HTML5 version that is displayed in the browser. If they have to register to get a local copy of the paper, instead of paying $35 to get it from a journal, they don't mind that. Over 2 million academics have signed up, and we have about 4.5 million monthly unique visitors, so we have a fair amount of data on this.