Facebook?

Not exactly sure what you're asking. I suspect that in this case, some people have the setting where they don't allow strangers (nonfriends on Facebook) to message them. It doesn't matter if you have mutual Facebook friends. And the only way to be allowed to message them is to friend them in the first place.

People don't trust such incursions. You probably should get the friend to alert them and ask if they want to see your messages.