Whether you remember it or not, you may have once granted Facebook permission to store the contacts in your phone's address book. If you did, Facebook is continuously uploading these contacts to its servers.
You can find out if your phone is doing this, and stop it pretty easily.
Here's how to do that on Facebook for Android:
Open the Facebook app on your Android phone.
Tap the menu button on the top right of the screen.
Select "Settings & Privacy" near the bottom of the list.
Select the "App Settings" option.
Disable the "Continuous Contacts Upload" feature.
How to do it on Facebook for iPhone and iPad:
Tap the menu button in your iPhone or iPad Facebook app.
Select Account Settings.
Select "Upload contacts."
Turn the option off.
How to remove contacts that have already been uploaded:
First, make sure that you've followed the instructions above.
Visit Facebook's contact management page from your web browser.
Select "Remove all contacts."
This article was first published on CNBC