>>If there is no solid proof of e.g. the company selling to someone they knew is a criminal, there's nothing to be done, legally speaking.

If you look at the article it has examples found of the company employees explicitly saying they are meeting with criminals so to play it safe. It doesn't get any more "solid proof" than that.

>>are those networks / manufacturers / anyone but the criminal responsible for what is talked about?

No, but again - read the article. There are examples of their employees saying that a client of theirs was arrested so they proactively wiped their phone - that could be interpreted as knowingly destroying evidence. They did end up changing this policy to not wipe phones of people who have been arrested, precisely because of this concern.

>>Yes the seller could reasonably assume their stuff was used by criminals, but so can Signal, Whatsapp, Messenger, anyone offering (encrypted) communication

The difference is most likely in how it's advertised and sold. Whatsapp is a free app that anyone can use, Facebook can reasonably claim that they don't advertise to criminals or encourage illegal use because the app is free to anyone. The owners of this app made it paid and they actively pursued clients they knew were members of criminal rings. Whether that passes the threshold for holding the company liable - that's for courts to decide. But that's generally where I think the line is. Anyone can make and sell a knife, but start selling knives(knowingly) to gang members and you're going to be in trouble even though selling a knife isn't illegal in itself.