Many people are Facebook friends of their employers, churches, or other organizations. Now imagine you owe (or at least a collector thinks you owe) $362 for an unpaid car loan and your employer gets a call from a known debt collector asking them to tell you to return their call. Immediate privacy violation! That's why the FTC doesn't allow debt collectors to share that information publicly.

Now, unlike a phone call at random, in the case of a Facebook contact, it would be pretty easy to figure out that the stranger who messaged you is trying to collect on a debt.