The new head of Britain's electronic eavesdropping agency has accused internet firms of being "in denial" of the role their networks play in terrorism and demanded they open themselves up more to intelligence services.
GCHQ director Robert Hannigan said they had become the "command-and-control networks of choice" for a new generation of web-savvy criminals and extremists, such as Islamic State (IS) jihadists.
Better arrangements had to be developed to allow security and intelligence agencies to police online traffic, he said in an outspoken article for the Financial Times, warning firms that their users did not want their social networks used "to facilitate murder or child abuse".
"GCHQ and its sister agencies, MI5 and the Secret Intelligence Service, cannot tackle these challenges at scale without greater support from the private sector, including the largest US technology companies which dominate the web," he wrote.
"I understand why they have an uneasy relationship with governments. They aspire to be neutral conduits of data and to sit outside or above politics. But increasingly their services not only host the material of violent extremism or child exploitation, but are the routes for the facilitation of crime and terrorism.
"However much they may dislike it, they have become the command-and-control networks of choice for terrorists and criminals, who find their services as transformational as the rest of us.
"If they are to meet this challenge, it means coming up with better arrangements for facilitating lawful investigation by security and law enforcement agencies than we have now."
The question of state surveillance of communications was thrust under the spotlight by the exposure - by US whistleblower Edward Snowden - of secret mass data collection programmes run by the US and UK authorities.
Mr Hannigan conceded that GCHQ had to "show how we are accountable for the data we use to protect people" and was "happy to be part of a mature debate on privacy in the digital age.
"But privacy has never been an absolute right and the debate about this should not become a reason for postponing urgent and difficult decisions.
"To those of us who have to tackle the depressing end of human behaviour on the internet, it can seem that some technology companies are in denial about its misuse," he went on.
"I suspect most ordinary users of the internet are ahead of them: they have strong views on the ethics of companies, whether on taxation, child protection or privacy; they do not want the media platforms they use with their friends and families to facilitate murder or child abuse.
"I think those customers would be comfortable with a better, more sustainable relationship between the agencies and the technology companies."
Mr Hannigan cited the fact that IS stopped short of showing actual beheadings in gruesome videos announcing the killings of Western hostages - incuding British aid workers David Haines and Alan Henning - as proof of extremists' increasing expertise in online propaganda.
"They have realised that too much graphic violence can be counter-productive in their target audience and that by self-censoring they can stay just the right side of the rules of social media sites, capitalising on western freedom of expression," he said.
Smartphone and other mobile technology had " increased the options available exponentially" to conceal terrorist activity, he said, including applications " proudly advertising that they are 'Snowden approved'."
Emma Carr, director of Big Brother Watch, said: "It is wholly wrong to state that internet companies are failing to assist in investigations.
"The Government and agencies have consistently failed to provide evidence that internet companies are being actively obstructive.
"These companies have consistently proved through their own transparency reports that they help the intelligence agencies when it is appropriate for them to do so, which is in the vast majority of cases.
"Public debate on this issue would make the country stronger and more unified, yet we have so far failed to achieve this in the UK. Perpetuating falsehoods about the nature of relations between internet companies and the intelligence agencies is certainly not going to help."