Social networking web sites or Social Networks for short (SNs) have become an important web service with a broad range of applications. In an SN, a user publishes and shares information and services. The authors propose a utility function to measure the rational benefit derived by a user from the user participation in an SN, in terms of information acquired vs information provided. They show that independently and selfishly maximizing this utility leads users to \"Free-riding\", i.e. getting information about other users and offering no information about them. This results in SN shutdown (no functionality).