Anyone knowledgeable about this?
Yes. Material flagged by users can be contributed through 'channels' which will result in updates to PhotoDNA's databases. This is somewhat laggy, so your bit about extremely recent content is on point. But if a recipient would flag an image as inappropriate then whichever service you are using should have a channel to LE to pass on suspect images and to take them down pre-emptively.
Reviewing such images is extremely stressful on the reviewers and PhotoDNA does a fantastic job in handling a very large amount of material automatically. I've only once come across one file sharing service that put up resistance against implementing it (for the most absurd reasons) usually parties are very happy to have a good chunk of this nasty aspect of running anything that shares online images or video taken care of.
When I ran Camarades.com PhotoDNA did not yet exist, I would have been super happy if it did. Some of the stuff that came up for review after user flags was just terrible and even 20 years later those images have not left me. We had a hotline to LE and had a laggy log to be able to recover the uploaders. This was also one of the reasons I eventually shut down files.ww.com, the number of jerks out there was just too large to deal with.
One article about this:
https://www.theguardian.com/technology/2017/may/04/facebook-...
I knew one LEO here in NL that was at the forefront of tech and the amount and gravity of abuse it generated, he couldn't deal with it from a psychological point of view. And those are trained and assisted, the likes of FB are much less likely to vet their employees properly because the burn-out rate is so high.
> The appearance of the images on Twitter was striking because they had been previously flagged as child sexual abuse material, or CSAM, and were part of databases companies can use to screen content posted to their platforms, the researchers said. “This is one of the most basic things you can do to prevent CSAM online, and it did not seem to be working,” Thiel said.