The critical mass of a network refers to the point at which the value produced by the network exceeds the value of the product itself and of competing products. This can happen at different times depending on the type of a network.

For example, physical direct networks such as telephones gain critical mass quite early on. As the chairman of AT&T pointed out back in 1908, "a telephone - without a connection at the other end of the line - is not even a toy or a scientific instrument. It is one of the most useless things in the world." Since one telephone without any connections is utterly worthless, a telephone network with even two users has sufficient value to exceed the inherent value of a single product on its own.

Contrast that with a platform network like Windows or iOS. The value of the Windows operating system, even without any programs or applications, is quite high on its own. Only after the network of users and developers has grown quite large does the value of all the third-party programs, plus the value of the interoperability with other users, exceed the value (for users) of the Microsoft programs by themselves.

Most products with network effects must ultimately reach critical mass in order to fully take advantage of the defensibility provided by their network effects. Before the size of the network reaches critical mass, the product remains quite vulnerable and may not have much value to users. For such products, the challenge is often to build enough initial value to incentivize early adopters to start using the product even before the network effects value has kicked in.