First let me start by saying this article is for the iOS (iPhone and iPad) version of Facebook Messenger. I haven't had a chance to look at the Android version yet.
There is an article going around today spouting out some scary things about Facebook Messenger. This article claims that with the new version of Facebook Messenger Facebook has stepped over some serious privacy lines with their "new" permissions. I have chosen not to link back to the original article here as the purpose of this persons article was to draw people to his site so he could collect ad revenue. I want to however take the next couple sentences to clear things up.
First I want to clear up how an iOS (iPhone and iPad) app works. Apps are kept inside their own little space on iOS. No app can reach out and just do what it would like. Instead apps must ask for permission for each action that reaches out beyond the app itself. This prompts you with the little dialog box you're probably familiar with. Let's take accessing your contacts, a permission that Facebook asks you for because it uses those numbers that people have already registered to show you who of you friends is also on Messenger. Facebook prior to being able to access these contacts must pop up a dialog box asking for your permission to do so. NO app can grant itself permission to access something outside itself. The same is true of permission to access other things such as camera, location, and calendars. NOTHING can be accessed without you actively giving it permission. You also have the option to deny it right there. Denying it permission may inhibit an activity on the app (say not allowing it access to the microphone would not allow it to record sound while taking a video) but that is always your choice, not the apps. If you have ever wanted to see what you have given permission to or feel like taking permission back you can go to...
Settings--> Privacy and then chose any permission you would like.
Bottom line is this. IOS from it's very creation was designed to protect the user. An app can only do what you allow it to do. Yes, Facebook Messenger is completely fine. We're also going to ignore the notion that Facebook actually cares to creep on you.