How does Skype make money?

It was a big question for me? Paid calls are only a fraction of free ones, there is no visible advertising,... so, how do they make money?

This link is a bit old, but (thanks Gaurav, for sharing) it answers some of the questions.

http://voiceontheweb.biz/2009/03/skype-business-model-revealed-at-ebay-analyst-event/

Now, we can discuss its sustainability when competition is looming...