The only way they might run afoul od COPPA is if they were alerted that a particularuser was underage and they failed to remove that account from their network.
There is probably a sweet spot of mid-sized apps that have particular enough user profiles, but a lot of that kind of user, that are worth building custom campaigns for.
But since a more focused search will likely yield more useful recommendations, buzz.com will also be able to suggest experts for thatparticular topic based on user "favorites" and comments.