This Wall Street Journal article says that many of the most popular of the 550,000 Facebook apps (!) have been transmitting identifying information about users and their friends to dozens of advertising and Internet tracking companies.

“The apps reviewed by the Journal were sending Facebook ID numbers to at least 25 advertising and data firms, several of which build profiles of Internet users by tracking their online activities.

Defenders of online tracking argue that this kind of surveillance is benign because it is conducted anonymously. In this case, however, the Journal found that one data-gathering firm, RapLeaf Inc., had linked Facebook user ID information obtained from apps to its own database of Internet users, which it sells. RapLeaf also transmitted the Facebook IDs it obtained to a dozen other firms, the Journal found.

RapLeaf said that transmission was unintentional. “We didn’t do it on purpose,” said Joel Jewitt, vice president of business development for RapLeaf.”

Update: Facebook responds.