Monopolies are illegal in an economy but Facebook, Google, Apple, and Amazon are legal in spite of being monopolies. Google controls 89 percent of the search engine market, Facebook has 2.1 billion active users, Amazon controls 57 percent of the e-book market, and Apple is the largest company in the history of the world by market capitalization.

A monopoly can be created in two ways, prevent competition from entering the market or prevent customers from switching to competitors.

Competition is inevitable in a profitable market. Therefore, creating a defensible business becomes more important than scaling a business.

A business can prevent competition by increasing its value to customers. It needs to continuously innovate to avoid competition. Snapchat is on the innovation treadmill. 

However, innovation is unpredictable. Instead of innovating continuously to provide benefits to consumers, a business can use network effects to increase benefits to consumers.

Apple, Google, Amazon, and Facebook highly depend on the network effect. Each additional user benefits the existing user base. Since Whatsapp has acquired 600 M users, even if a startup captures 100 M users in a particular network, it is very difficult to dislodge the rest 500 M Whatsapp users. Snapchat ran into a similar problem, it captured teens but was not able to dislodge mainstream Instagram users. Networking benefits allow you to be a monopoly legally.