Dubbed "viral marketing", company's use 'Refer a Friend' competitions (e.g. send a postcard, quizzes, games) to get their users to tell other people about the site or to get them to register for newsletters/services etc. Incentives of prizes add to their attraction to participate in these competitions.
Entry to the competition is often achieved by providing email addresses of 3 or 5 friends. Further entries are often gained by referring more friends. Alternatively, referring a friend can gain you additional entries in some competitions.
Be careful when referring your friends and family in these kind of competitions. Make sure you have their permission.
