Facebook has been criticised for its handling of reports about sexualised images of children on its platform.
The chairman of the Commons media committee, Damian Collins, said he had "grave doubts" about the effectiveness of its content moderation systems.
Mr Collins' comments come after the BBC reported dozens of photos to Facebook, but more than 80% were not removed.
They included images from groups where users were discussing swapping what appeared to be child abuse material.
When provided with examples of the images, Facebook reported the BBC journalists involved to the police and cancelled plans for an interview.
the BBC used the report button to alert the company to 100 images which appeared to break its guidelines. They included:
◾pages explicitly for men with a sexual interest in children
◾images of under-16s in highly sexualised poses, with obscene comments posted beside them
◾groups with names such as "hot xxxx schoolgirls" containing stolen images of real children
◾an image that appeared to be a still from a video of child abuse, with a request below it to share "child pornography"
Of the 100 images only 18 were removed.
According to Facebook's automated replies, the other 82 did not breach "community standards". They included the apparent freeze frame.
Facebook's rules forbid convicted sex offenders from having accounts.
But the BBC found five convicted paedophiles with profiles, and reported them to Facebook via its own system. None of them were taken down.
The BBC first asked Facebook for an interview about its moderation system in late-2015, and repeated the request following this follow-up investigation.
The social network's director of policy Simon Milner agreed to be interviewed last week, on condition the BBC provided examples of the material that it had reported, but had not been removed by moderators.
The BBC did so, but was reported to the UK's National Crime Agency as a consequence.
Mr Collins said this was "extraordinary - because you're trying to help them clean up their network, from material that shouldn't be there".