This has nothing to do with the separation of the Messenger app from the core Facebook app. When messaging was packaged as part of the core Facebook app, the functionality was the same. There is nothing Facebook can do with two separate apps that they couldn't have done with a single app. You can't tell me that "Facebook Messenger spies", but that "combined Facebook and Facebook Messenger apps wouldn't spy".
And to be clear, "spying" simply means granting the app those privileges. You could complain about Apple's or Android's approaches to approving individual privileges, but it's not like Facebook is doing anything super illegitimate that users haven't had to permit - even if the reason for granting those privileges isn't fully disclosed.
Messenger was broken away from the main Facebook app because it was the principle use case for Facebook users - messaging - and by splitting the function off, Facebook was able to be even more intrusive, since the assumption is that "its just a messaging app, it doesn't do anything more than that like the main app" - whereas, it actually does a lot more than that behind the scenes in order to gain access to data that Facebook wants to know about you.. such as listening on the microphone to gain clues about where you are, who you are with, and what you are doing.
>App privileges
Sure, just like the NSA isn't spying, since it has permission from Congress. You won't ever know what it does with the data - because of course, those rights are privileges and not granted to you, the individual - but it doesn't make the net result any better for those of us who care about privacy and controlling our own lives, un-hindered by corporate policies requiring the mass usurpation of our individual rights.
And to be clear, "spying" simply means granting the app those privileges. You could complain about Apple's or Android's approaches to approving individual privileges, but it's not like Facebook is doing anything super illegitimate that users haven't had to permit - even if the reason for granting those privileges isn't fully disclosed.