You clearly don't understand what "big brother" is.

"Dangerous" speech has propagated via numerous means for millennia. For example: ~240 years ago people were fomenting revolt using printing presses.

Big Brother is an all-seeing, all-knowing, totalitarian leader of a society that wields its power for its own sake, using surveillance as a tool towards that end. That's what some massive corporations effectively are today. People think today's Big Brothers are benign because their mission statements seem just or useful, but as I've shown above, there is significant potential for collateral damage.

You're right, and we've needed to control those means numerous times over the millennia. Sometimes just to keep the powerful in power, sometimes to check the powerful, and sometimes to check the surging wave of popular revolt.

Rwandan genocide via radio

https://www.concordia.ca/research/migs/resources/rwanda-radi...

Remember the Maine! via newspapers

http://www.ushistory.org/us/44c.asp

Foreign government's use of social media is the most alarming thing in my opinion...

Revealed: Israeli Military Monitors Social Media, Blogs and Forums in Search of 'Security Leaks': https://www.haaretz.com/israel-news/.premium-revealed-israel...

Jewish Internet Defense Force: https://en.wikipedia.org/wiki/Jewish_Internet_Defense_Force

How does encryption work for group chats on WhatsApp? Could Facebook have moderated these messages to prevent the attacks or is the content still hidden to them?

WhatsApp supposedly uses signal protocol, which implies that the messages are e2e encrypted and fb doesn't have any visibility to the contents. Of course like always it is difficult to say what capabilities fb really has.

https://techcrunch.com/2014/11/18/end-to-end-for-everyone/