I agree with mets. A good example is a thread from about a couple months ago, where someone asked if it would be ok to ask a former transfers coach about that players potential. A lot of people thought it was ok, as there was no agreement, and anyone can ask. A lot of people thought it was not ok because you gained access to information not already available to you, through communication with another coach. Seble stepped in and said NO, it's not allowed behavior.
I think that is a great example because on first blush, it sounds very harmless. But through the interaction (not agreement) of two coaches, one coach can garner an advantage over another coach.
I think the line is drawn where an advantage can be claimed for a coach by their activity. However, this has to be weighted against the fact that most coaches are most involved in the game during recruiting and that is when they'd most naturally want to post. So its not black and white, but when any significant advantage can be gained, it is against fair play, IMO