How does Skype make money when everyone is on Skype?

Andrew at Phanfare was answering a completely seperate question when he noted what I think is a really interesting consideration.

"What Skype sells, mostly, is the ability to call in and out of the
traditional phone network (SkypeIn and SkypeOut). That makes strategic
sense. The free users don’t cannibalize the sale of the SkypeIn and
SkypeOut (well maybe to a very small degree since the more Skype users
there are, the fewer people outside of Skype there are to call), and in
fact increase the pool of people who might buy the upsell."

So, when we all have free Wifi or WiMax or WiWhatever and we're all using Skype to call each other...

...  How does Skype make money?

I mean, the whole business of Skype is based on the concept that there are people not on Skype, because you only pay to receive or make calls from outside the Skype network to regular phonelines.

Can you think of another example of a business where converting every last human being on the face of the earth to customers would drive revenues to zero?