It applies to community networks in that the user of the community network must be getting value from that network, even though they are not being charged for it. People will not use one network that gives them less value than another network. This value can be positive or negative. An example of negative externalities is pollution and cars.
Pollution is an indirect product of driving a car.
The negative value that you get from polluting is small enough, and indirect enough that the value of having your car is greater than adding a bit of pollution to the air. You concentrate on getting the car, not the pollution. Network externalities are one type of positive externality. Consumers get positive value from other people using the network. This is from knowing that there will be support and knowing that the information will be continuously updated.