Illegal under which laws. Qatari law, which outlaws homosexuality? Russian law, which prohibits referring to the "invasion" of Ukraine? Or do you assume everyone around the world is subject to US law, so it's fine for all our messages to be intercepted and checked for compliance with US law?
> A noob question, how does signal know/prevent use of its app for illegal/criminal activities?
If you allow for privacy you allow for illegal usage. There is no way around it.
> Larger question here would be - Do governments and security agencies need to keep a tab on social media to check for illegal activities
They can do it old fashioned way. Also every compromise on security can be used by bad actors as well, and in some cases government is that bad actor, even in 1st world countries.
Larger question here would be - Do governments and security agencies need to keep a tab on social media to check for illegal activities