What about the annoyance factor? "Hey why are these strangers contacting me and asking me to send you messages to contact them?! Do I look like your personal secretary??"
Yes, I agree there's an annoyance factor. In all honesty, what do you think should happen when someone isn't paying a debt they owe? Can you think of a reasonable way to get hold of someone who's refusing your phone calls that doesn't involve potentially annoying friends and family?
Here's the thing. YOUR debt collectors don't have the right or the privilege to harass ME over your debts, just because we're linked as "friends" on facebook or whatever social network, even if they don't tell me the reason why they are pestering me to deliver messages to you. If someone decides that they do, you're no longer my "friend", AND I'll likely stop using Facebook / social network I am tired of getting other people's spam on.
Once you file suit, it's the process server's job to get ahold of you. They have less restrictions. And you can tack on court costs to the suit you bring -- and then use the grievance you win to garnish their wages.
It sounds easier than it looks, but that's how you get your money as a debt collector. The ones you can't find are probably just as like to be no shows in court. Once you have a greviance, it's easy to garnish. All that involves is contacting their employer.