As privacy blunders go, Facebook's accidental leaking of User IDs to advertisers and data firms is pretty insignificant.
Some third-party apps, including Farmville, were inadvertently passing User IDs along because of a Web standard called "referrer" that reveals your previous URL when visiting a webpage. Facebook referrer URLs include User IDs, and these are in turn linked to your name and any other information you make public.
So what? As Michael Arrington pointed out on TechCrunch, the sharing of your information goes with the territory of the Internet, especially with services that use real names, like Facebook. In this case, some advertisers might learn that you use Facebook, and, at worst, that means they'll be able to deliver more relevant ads to you. It is not the end of the world.
Marshall Kirkpatrick at ReadWriteWeb even notes how RapLeaf, one of the data firms the Wall Street Journal calls out for collecting user data, puts the information to good use: A Gmail plug-in called Rapportive uses the data to create rich profiles of your e-mail contacts. Again, this is all using publicly available information.
Besides, data collection is not unique to Facebook. Adam Penenberg, a journalist and NYU professor, noted that some ad hackers use spiders to gather information from Facebook and Digg, and even hack iTunes to create marketing dossiers on individuals. Companies like MySalesHero can generate thousands of marketing leads from a single zip code, as Jeff Jarvis pointed out.
I'm not saying that the transferring of User IDs isn't a mistake and shouldn't be stopped. Facebook says that it doesn't share personally identifiable information to advertisers, and that third-party apps are prohibited from doing the same. If that's the policy, it needs to be enforced.
But if you're really worried about privacy on Facebook, the bigger issue is an increasing number of opt-in services, designed to publicly share more information than you previously allowed.