Who's doing the dirty work of cleaning up the internet?

Somewhat overlooked in Facebook's announcement this week that it will launch a new dating service was CEO Mark Zuckerberg's repeated commitment to have 20,000 people screening the platform for unwelcome content by the end of this year.The content Facebook says it wants to screen out ranges from fake news to violent livestreams.

But, those who've been investigating just how that content is screened out and, more precisely, who is actually doing itnoted Zuckerberg didn't specifically say if those people would be employed directly by Facebook, or whether the work will be contracted to third-party contractors, as much of it is now.

"It's not just about the quantity [of people].It's also about the quality," said Hans Block, a Berlin-based documentary filmmaker. "You have to train really well-educated people, a diverse number of people doing that job.Not just low-wage workers."

Block and Moritz Riesewieck's recently finished filmThe Cleanersinvestigates how companies like Facebook, Google and Twitter are dealing with the flood of violent and abusive content flowing onto their platforms.

Companies do not offer psychological support

They found much of the "cleaning" has been contracted to third-party companies in Manila, Philippines, who employ young workers to watch as many as 25,000 flagged images a day to decide what should be deleted.

"There are thousands of young Filipinos sitting in front of a desk, and they reviewchild pornography, beheadings, terrorist videos, violence and all that cruel stuff," Block told The Investigators this week. "That has an effect on your mental health."

And the companies do not offer psychological support, he said.

• Watch the full interview with Hans Block andMoritz Riesewieck onThe Investigators, Saturday at 9:30 p.m.ET and Sunday at 5:30 p.m. ET onCBCNews Network.

Block and MoritzRiesewieckbegan their project three years ago. Finding out how the work was being done was a difficult task. "The new tech companies are incredibly secretive," Block said.

They gradually made contact with workers who revealed to them what the job is like.

"We chatted [online] with a lot of them and they gave us insights into their actual work," Riesewiecksaid.

The documentary illustrates the split-second choices the workers say they are asked to make about whether a video violates guidelines around sexual assault or child abuse, for instance.

Riesewiecksaid it wouldn't have been enough to simply send a hidden camera in with one of the screeners to document the work.

"It's about digging deeper," he said. Some of the workers they connected withended up leaving the company and worked with the filmmakers to recreate their experiences using a vacant Manila office space.

"We could transport the feeling of somebody sitting on the 20th floor, high above the city of Manila, and getting all the material of the world on screen, and how overwhelmed somebody must feel being in that position," Riesewiecksaid.

No response from companies

Block says they've offered those who took part in the documentary the services of a psychologist.

ButRiesewiecksaid in many cases, even where it appeared the work had taken a psychological toll, "most of them are actually quite proud about what they do because they told us 'we do one of the most important jobs of theinternet, and the world should know what we are doing. Because without us, theinternetwould be a mess.'"

As to Facebook, Twitter and Google, all three of whom are featured, Riesewiecksaid they made repeated attempts to get the companies to comment.

"There was no response," he said."We even sent them the finished cut and, no ... no reaction."

Also this week on The Investigators with Diana Swain: CBC Ottawa journalists David Cochrane and Lisa Laventure talk about their investigation into the role of a federal prosecutor in the extradition of a Canadian man.