Many were not started to make a profit. Many were started for an IPO or roll up. Media Companies provide access to the Internet or organize on-line information for users. Content Companies provide specialized information for the customers. Each of these types of companies receive their revenues from advertising and marketing deals with companies that are selling products through the Internet.
Revenues come from a percentage or orders placed by consumers through the Internet.
Most of their costs are fixed and don't change, whether they have 100 customers to 100 million customers. Each additional customer flows directly to the bottom line. Convenience is the major attraction. People's lives are getting busier. In bad weather or at the end of a long work day they can order what they want over the Internet.