Most free web services plan to make money by the following:
Advertising model – Know as much as possible about the user and bring targeted ads.
Freemium model – sell a free product and plan to convert some of them to a paid plan.
Limited period promotion – Start with the free product for a promotional initial period and plan to charge it later. For instance, 37 Signals provides a free 30-day trial offer for most products and then charge if you use later. This is a tough thing to master these days.
