The problem with a paywall is that the core of what makes Facebook successful is the fact that "everybody" you know is on there. A freemium model might be the

The problem with a paywall is that the core of what makes Facebook successful is the fact that "everybody" you know is on there. A freemium model might be the only feasible paywall option. They have to be careful not to alienate the large percentage of users that will never upgrade. LinkedIn is a good example of how to do things right, however, their audience of more professional users is a bit different