Facebook loves doing unexpected things with your data. Now it's culling numbers from your phone and adding them to its online database to "help you" find contacts. Don't trust Facebook with your mum's number? Here's how to fix it.
If you're syncing, Facebook's iPhone app will pull all the numbers in your phone's contact file and upload them to Facebook. While it notes that these numbers are only visible to you, Facebook has a way of, let's say, shifting its policies. Worse, for many users these numbers are already in its database, and if you're not comfortable with that (and I'm not) you're going to need to manually purge them.
First, you want to get your phone straightened out so it doesn't sync anymore.
Using an iPhone, launch the Facebook app in your iPhone and select Friends. Tap the sync button on the top right and tap sync contacts. Turn off syncing.
If you're on an Android phone, go the main menu, and click on Settings. Under Other Settings, select Sync Contacts. Select Remove Facebook data.
Now you need to go to Facebook and actually purge any contacts that have already synced. You should go to this page, and select Remove. Bang. They're gone.
Take that, Mark.