I’d have to partially agree here. I could show you my private messages inbox, over half of the recent messages being filled with “Join my rank of allies in Dueling Blades!” Clearly, I’m already playing the game. =P
Personally, if I think a game is good, I’ll talk about it in chatrooms in my own words. Having a computer do it for me just seems impersonal and rude, so I never end up using such features myself. Other people using them on me, I’ve come to terms with — mostly I just skip over these messages since I don’t pick up new games this way.
It isn’t necessarily bad to have a feature like this but I’d wager most friend networks on Kongregate often hang out in the same chatroom together, and share games in those chatrooms. I’ve come across a few hidden gems that way, and it’s considerably less annoying than an automated private message. ;)