The general idea behind all of the social networking sites is that people are invited to join by existing members who think they would be valuable additions to the community.

Question

Quizplus · Accepted Answer

This is generally true for most social networking sites. Existing members can invite friends or others to join the community, thus helping it grow. However, some social networking sites may also allow for self-registration without an invitation.

The General Idea Behind All of the Social Networking Sites