There is a popular Social network site that I saw today has Google ads running in their IM windows. Its a IM that is a popup in your browser .. firefox etc. I don't want to get them in trouble but is that breaking googles rules ?
i'm too lazy to read back thru the TOS and polices, but i recall that displaying ads within a chat is NOT permitted.. however, if the social network site is "popular" as you mention, it might be a premium publisher and may have different rules/TOS..
Since the site has it's own "IM" and is that "popular", I'm going with Tearabite's suggestion that they may be Premium, in which case, probably no violation.