There is a command associated with social spy. I don't recommend using it. If you absolutely must for any reason, keep in mind that what's said in private messages stays in private messages. They're called private for a reason, and you and everyone else has no business sharing what they've seen players messaging to one another.
Also keep in mind that players conversing about joining other servers is none of your business, and not advertising. If a player is asking "hey wanna come to my server" it counts as advertising. If a player is telling the player "go to x server it's better than this one" it counts as advertising. Otherwise, ignore it.