Soaring numbers of alleged crimes on Facebook and Twitter are being investigated by police, new figures have revealed.
Thousands of complaints involving social media were reported to forces across the UK last year, including allegations of sexual offences, threats to kill and harassment.
The National Police Chiefs' Council (NPCC) said police and internet providers had a "responsibility" to protect people online, while Facebook insisted it did not tolerate abusive behaviour and urged users to report illegal activity.
A total of 38 out of 45 forces reported a rise in the number of crime reports where Facebook appeared in 2014, according to figures obtained by the Press Association under the Freedom of Information Act.
Twenty-four forces said they also received more crime reports which mentioned Twitter last year than in 2013.
Britain's largest force, the Metropolitan Police, received 1,207 crime reports which mentioned Facebook in 2014, up from 935 in 2013 and 997 in 2012.
There was also a sharp rise in crime reports which included Twitter, from 105 in 2013 to 138 in 2014, the Met revealed.
Greater Manchester Police (GMP) said Facebook appeared in 959 crime reports last year, up from 512 in 2013 and 451 in 2012.
Last year's complaints included 371 allegations of harassment, 38 threats to kill and eight rape allegations involving girls under the age of 16, GMP said.
Meanwhile, Staffordshire Police reported a 40% rise in crime reports which mentioned Facebook after receiving 1,269 in 2014, including 326 complaints of harassment, 13 rape allegations involving girls under 16 and 38 threats to kill in 2014.
Twitter appeared in 56 crime reports in 2014, up from 48 in 2013 and 25 in 2012, the force revealed.
Detective Superintendent Scott Jones, of Staffordshire Police, said: "Without trawling each crime it would be difficult to establish what context Facebook or Twitter played in each crime.
"However, it would be fair to say that like many other forces across the country we have seen an increase in offences online but the majority still go unreported."
Superintendent Paul Giannasi, of the NPCC's hate crime working group, said: "We are working with industry partners and others to try to tackle the level of abuse, harassment and other offensive content on social media, but we have to balance the right to free speech with the need to protect individuals from targeted abuse.
"There have been a number of successful prosecutions against people posting offensive and abusive messages, including under new legislation making revenge pornography illegal. In some cases this has led to the offender being imprisoned.
"There is a responsibility on police and internet providers to protect people online.
"Anyone who feels that they are being harassed on social media should report it to the police via the dedicated True Vision website so that we can investigate it fully."
A Facebook spokesman said the company responds to appropriate requests from police to provide information about illegal activity to help ensure the site remains a safe place.
The social networking giant said it does not tolerate abusive behaviour and operates a "real name" policy so that people are accountable for their actions.
Facebook is "highly self-regulating" and the website encourages its 1.3 billion users to report content they find upsetting or which breaks the site's community standards, the spokesman said.
Many police forces pointed out that even though Facebook or Twitter appeared in a crime report, social media may not have been used as a tool to commit an offence.
Kent Police, which had the highest number of crime reports to mention Facebook last year, reported a sharp fall from 3,683 in 2013 to 1,608 in 2014.
The force said the use of the words Twitter or Facebook in a crime report "does not necessarily mean that the crime has anything whatsoever to do with social media" and it estimated that 3% of total offences referred to online crime.
The Police Service of Northern Ireland said Facebook appeared in 682 crime reports in 2014, up from 451 in 2013 and 226 in 2012. There were also 14 reported crimes on Twitter last year, 15 in 2013 and eight in 2012.
South Wales Police said 586 crime reports mentioned Facebook last year, including 382 complaints of violence against the person offences and 32 allegations of sexual offences.
Meanwhile, West Midlands Police said the "closest information" it could provide was to issue figures for harassment allegations from material posted on Facebook and Twitter.
The force revealed 314 complaints of harassment on Facebook were reported in 2014, compared with 324 in 2013 and 203 in 2013. Twenty complaints of harassment on Twitter were reported in 2014, compared with 30 in 2013 and 10 in 2012.
Police Scotland did not provide figures because the force said the cost of retrieving the information would exceed the limit set out under freedom of information laws.
A Twitter spokesman declined to comment on the figures.