By Tom Cheshire, Technology Correspondent
Many social media users shared the horrific video of James Foley's murder at the hands of militants, but others called for a self-imposed media blackout on all IS imagery after the journalist's death.
"Don't watch it, don't share it," they wrote, adding the hashtag #ISISmediaBlackout.
Online every Twitter or YouTube user is his or her own executive editor. Material that wouldn't be published in a newspaper, or run on TV, is often shared.
But the IS blackout campaign also started with individual users, partly out of respect for Mr Foley and his family, partly to try to deprive IS of publicity.
A twitter user called @libyaliberty posted: "You know what I think? And I know how crazy this sounds, but we need an #ISISmediaBlackout. Amputate their reach. Pour water on their flame."
Others agreed. Soon the topic was trending, and celebrities such as Mia Farrow have echoed the call.
Sky News has not shown the images from the video.
 IS has posted numerous videos on social media sites
 IS has posted numerous videos on social media sites  Executive editor John McAndrew explained: "This video is deeply graphic, and what's taking place is apparent from the first frame. It's not something we need or want to show to tell the story. It would be deeply upsetting and distressing, for anyone watching."
But Sky News has played audio from the video, as the killer's British accent is relevant to the story.
The Met Police put out a statement saying it wanted to warn the public that sharing or even viewing extremist material may constitute an offence under the existing terror legislation.
Its advice was for members of the public not to distribute videos like that showing Mr Foley's death.
The early years of social networks were boom time for unfettered liking and sharing.
But, as these platforms have matured, so have their users. Many are now exercising editorial judgement just as mainstream news organisation do. These social networks are now mainstream themselves.
That's good news. Networks like Twitter and YouTube are loath to do anything unless users demand it. YouTube, for instance, says it has "clear policies that prohibit content like gratuitous violence, hate speech and incitement to commit violent acts".
 IS has seized control of vast areas of northern Iraq and Syria
 IS has seized control of vast areas of northern Iraq and Syria  But it admits that it only removes videos violating these policies "when flagged by our users".
So, if users are becoming more critical, that means graphic videos are less likely to prevail on the site.
Twitter CEO Dick Costolo said: "We have been and are actively suspending accounts as we discover them related to this graphic imagery."
But it remains easy for suspended users to simply create another account, as many IS affiliated accounts do on Twitter and YouTube.
And new social networks spring up all the time, or make a point of anonymity and lax standards.
Ask.fm has become a haven for IS fighters and sympathisers.
The network was bought by American firm IAC, which promised to clean up its act or shut it down.
And when Twitter shut down IS accounts, jihadists migrated to Diaspora, a private social network. Private messaging services like Kik and Telegram may be impossible to police.
Part of the appeal of YouTube and Twitter was that the media could not black them out, for however sound an editorial reason.
Could the user-generated IS media blackout have a similar effect, driving adoption of less mainstream social media platforms?
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