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. A user is more likely to continue to use a product or service that allows them to have that type of reassurance. What this means for community networking This means that your community network will become more useful to your community as more people use it. It also means that if there are very few people using your community network, it is exceedingly unlikely that you will get more people to use it if there is another source of community information that is noticably being used more often.