Founded: 1999

Estimated users: 19 million

Addictiveness rating: 10

Curious as to what their school friends were up to Steve and Julie Pankhurst from Barnet created Friends Reunited. It quickly became popular and was sold to ITV for 12m and now has dating and genealogies spin-off sites as well. It has been identified as one of the many social networking sites that employers use to carry out research on potential employees to find out information that they would not disclose at an interview or on a CV.