I personally have not become friends with or met up with another client in this industry. It's not really on my TDL. I have on occasion, however, discussed providers in private messages on MERB and its sister sites. These typically-very-brief discussions are very surface-level and about, like
@DouMan said, matters that may help inform a booking decision (physique and services). The bulk of incoming messages are from users who saw a review I wrote and are reaching out because they have some follow up questions they want answers on before making a booking (I only respond to those whose vibes I don't find objectionable). I've never gotten a question asking about a provider's personal life or their sensitive personal information, and if I ever were to, I just wouldn't respond.
Echoing what
@DouMan said, most of what I hear about an SP's life secondhand was volunteered to me, without my solicitation, by a friend, acquaintance, competition, or outright enemy of the SP. if a client knew something about you that he shouldn't have, I'd venture to say that he also could've heard it from another SP you either know directly or with whom you share a common client or industry acquaintance. I suppose this is why a number of SPs I see deliberately keep their social circle in the industry very tight.
Edit: I'm not discounting the possibility that the client could've heard it from some client ol' boys club/network/grapevine. I'm aware that they exist and have discussed it to some length elsewhere on the board.