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Facebook

Despite Facebook’s popularity and ease of use, the terms and conditions of the social networking phenomena clearly state users must be over 13 years old to sign up for an account. This is due to the data protection laws in the U.S. regardless of the nature of the application. A common misconception is that this does not apply to the UK and that therefore it is acceptable for primary school children to be encouraged in its use.
The LGfL eSafety group would strongly advise London schools that this is not the case. The UK Safer Internet Centre (SWGfL) have very close working links with Facebook and at the Safer Internet Centre national eSafety briefings this year, the advice was as follows:

It is known under 13s use Facebook and this is an on-going problem because the environment is not suitable for younger pupils. Continuing to support and educate young people is critical, but we should bear in mind the legalities of under 13s on Facebook.
Any under 13 signing up to Facebook does so with a falsified date of birth, which is, technically, fraud, though law enforcement would never be used to deal with this problem.
Facebook has to comply with COPPA (Children's Online Privacy Protection Act of 1998). This means Facebook may not hold any details of young people under the age of 13 and they may not advertise to this group either. This legal restriction applies to Facebook regardless of where the child is signing up from (America or the rest of the world).
Conclusion: Facebook is for over 13s only.
13 to 18 year olds are treated as minors and have additional privacy aspects enforced. For further reading the UK safer Internet Centre has produced a useful guide to Facebook privacy settings which can be found here.