Earlier this month, Blizzard Entertainment (makers of World of Warcraft, among other successful computer games) decided that they would change their game forums from anonymous forums (i.e., you can't tell the identity of someone posting to the forums unless they tell you) to forums where comments are connected with a person's real name. After a firestorm of criticism (e.g., here) Blizzard spiked the program, at least for now. And the reason for going down this path, with its utterly predictable and embarrassing trajectory? Two words: Facebook Integration. Actually (as explained here) it's slightly more complicated than that, but what it boils down to is that Blizzard wanted to get in on some of that social networking magic, and giving everyone a single ID that was consistent across all of Blizzard's forums (and Facebook) seemed to be a good way to do it.
This is an old story, and one that often ends in class action lawsuits (e.g., Google Buzz, Facebook Beacon). Why do people keep doing this? My guess is because they see their existing user data as an asset, and they hate letting an asset go unexploited. However, that's the wrong mindset. The safest way to think of user data is as something that actually belongs to users, which they have allowed you to temporarily safeguard. The point of the user data isn't to exploit it, it's to allow a business to maintain its relationship with its users. If you want to integrate with Facebook - fine. However, the way to do so is going forward, collecting new data (with a clear explanation of what you're collecting the data for), and without degrading or changing the services provided for old users. True, at the outset, this seems much harder than leveraging an existing user base. On the other hand, many existing user bases don't like being leveraged, and going about things the hard way can take that into account, and avoid turning an existing base into a historical user base.