I have reasons to believe that it rather steals contacts from your phone rather than politely synchronizing them.
Once you install Facebook, it forces you to install Messenger aside. If you don’t it will keep sending you new message notifications but not letting you read them. So you install Messenger.
Next, when you set up Messenger, they show you some an eye-candy stuff, and convince you to enable uploading your contacts, call and text history. This is where the problem begins. I don’t want to upload my contacts to Facebook, but Facebook developers have set up things in a way that it will convince most users to do so. Bad.
So here’s my normal routine when installing Messenger:
Text anyone in your phone: NOT NOW
Is your number up to date? NOT NOW (why the hell do I have to let people look me up on my phone number?)
Skip phone number? SKIP (are you effing kidding me now?)
So even after completing these three steps, Facebook Messenger won’t stop bugging you with Contacts Upload and Friend Finder stuff. I remember I accidentally touched a ‘Get Started’ button somewhere and it immediately started uploading my contacts. Just like a fish put back into the water!
I got panic and turned off WiFi immediately, but what’s done is done. After meddling with the Facebook and Messenger apps, I was able to turn off ‘Continuous contacts upload’. You can find that in the Facebook app settings.
After a bit of struggle with Facebook on web, I was able to find the following URL. Now, this URL is very important. I even recommend you to bookmark and share it with friends.
Go to that URL, and you will see your past invitations (by any chance if you had previously shared your email password with Facebook) and other contacts stolen from your email account in one place. Scroll all the way to bottom and you will see a hyperlink saying “Remove all contacts”. Click it, and after a couple of confirmations, it will delete your imported contacts.
Towards the top of the same page, there’s a hyperlink saying “See contacts you’ve uploaded in Messenger.”. Click it, and then click the “Delete All” button. Don’t even bother reading the confirmation, just hit “Delete All” when it tries to convince you to keep them. That’s it!
Now I’m even more careful when working with the Facebook and Messenger apps.