I don't understand what difference it makes whether they are already monitoring certain part of something or not? The effect is still the same. You could also argue that some phones or comm lines provide a way to monitor or get alerts for certain cases such as terrorism etc, but not all lower level crimes.
And governments monitor people in the public, yet they are unable to prevent all crimes of happening. Should governments then not exist at all or the society in general? Should people not exist?
There are diminishing returns in the World to block any societally negative behavior. I think that is the key thing here in the argument. Police monitors the city, but they don't stop all bad wrongdoing. There's always underage kids drinking, there's drugs changing hands, there's all sorts of illegal things going on. Some are easier to prevent, some are worse and have worse impacts on the society, it's a spectrum of illegal, immoral or otherwise bad things going on everywhere at once. Just because there's something on spectrum somewhere doesn't mean the whole thing should be shut off or should not exist. Just because police doesn't get all the 17 year olds drinking a pint or two doesn't mean the police or the World should not exist.
You seem to have changed the object of the argument from phone companies to governments/the police, but are still insisting in making a comparison that I'm trying to explain is not valid. (The shift from phone companies to governments just adds another obfuscation step; if you abolish governments, then people will still murder each other. If you abolish Facebook, people won't be able to send death threats over Facebook).
I'll try to make my argument on why your comparisons are not valid one last time as clearly as possible. We have two arguments:
(1) "If phone companies can't remove all death threats, they shouldn't exist (even though nobody expects them to try remove any death threats)" -- you can replace it with "If the government can't prevent all murders, it shouldn't exist (even though nobody realistically expects the government to prevent all murders)" for your latest message.
(2) "If Facebook can't remove all death threats, they shouldn't exist (given that it's their policy to remove death threats)"
I think most reasonable people would disagree with (1). People can reasonably agree or disagree with (2), as they're doing here in this thread.
You seem to disagree with both (which is fine) but refuse to understand that the arguments against (1) are not the same as the arguments against (2). Because of this, you keep trying to convince people who are against (1) but for (2) that they're the equivalent. They're not, so you won't convince anyone like that.
My point is that, all of this is a spectrum of diminishing returns, fighting to stop some sort of behavior that some people may consider to be bad. There's no perfect analogy for anything. The only argument you can make is that Facebook is overall a net bad for the society - which it may well be, but then make that argument, rather than saying that because a platform can't stop people from selling drugs on it, it should be abolished.
And governments monitor people in the public, yet they are unable to prevent all crimes of happening. Should governments then not exist at all or the society in general? Should people not exist?