British police say that over 100,000 crimes have been linked to Facebook since 2005

"Crimes linked to Facebook reached more than 100,000 across the country in the last five years, it emerged yesterday.
Police chiefs in 16 forces revealed that 7,545 calls from the public since January were concerned with the social networking site.
Callers have alerted officers to alleged acts of terrorism, sudden deaths, missing pets and even firearms offences.
Facebook users have used to site to detect crimes ranging from terrorism to missing pets
Frauds, sexual offences and hate crimes were also reported to police, as well as a large number of malicious messages on the site.
The figures have been obtained by the Daily Mail via Freedom of Information Act requests to forces.
They compare to just 1,411 calls related to Facebook to police in the whole of 2005, when the network began to gain popularity."

Source:  Daily Mail, 14th December 2010