Could you buy Facebook ads that target the person's friends? All the data you need is there for the debt collector's taking. You wouldn't need to intimidate anyone or say anything hateful. The ad could be as simple as, "Get $5 in Facebook Credits. Have Your Friend Melanie Call Me." Not sure if that ad would actually work, just thinking out loud.
From Facebook's perspective, you don't want people to stop creating accounts (or quit using them) because of fear of debt collectors, but debt collectors are a business that could provide them a revenue stream.
It's already a known fact that some employers check potential hires on Facebook and I don't think that's had much of a chilling effect on the growth of their social network.