That means that any site that allows two people to communicate can be complicit -- be it forums, discussion boards or chat/messages. Criminals can easily adopt code words, or encrypt their plaintext communications in ways that a website would be unable to police. How about email? Are email providers also required to detect all criminal activity occurring in their emails?
Ultimately it's a massive burden placed on companies if they are required to read and sort through all communication occurring on their platform and it also necessarily removes any and all privacy from every communication platform. Do we do the same thing for cell phone manufacturers or telecom providers? What about monitors that display the messages or keyboards that let people type the messages soliciting illegal activity?
I think it's worth considering just how much influence Facebook has that many of the other entities you mentioned do not. I also think it's worth considering possible solutions to problems that Facebook may or may not be the direct cause of before jumping directly to "Welp! Slippery slope, cant do anything about it i guess!".
Ultimately it's a massive burden placed on companies if they are required to read and sort through all communication occurring on their platform and it also necessarily removes any and all privacy from every communication platform. Do we do the same thing for cell phone manufacturers or telecom providers? What about monitors that display the messages or keyboards that let people type the messages soliciting illegal activity?