[mbreese]

Not quite... we don't give Facebook or Twitter money - their advertisers do. So, their customers are the advertisers. What are they selling? Us - our eyes and attention.

So, we are using the site, but we don't directly give them money to do so. Instead, they get money by putting ads in our faces while we use the site.

Now, the Facebook/Twitter user can be a customer in this scenario, just not of Facebook or Twitter. We (typically) are customers of the advertisers who want us to buy a car or sugar water or a widget.

So, the store analogy isn't quite apt for this... instead you could think of it more like a TV network. They don't call the people who watch their shows customers or users, but rather "viewers". TV network's customers are the businesses that buy time to air their commercials.