For now, Facebook only allows users to opt out of its Beacon ads, which target your friends based on what you do on other websites, on a site-by-site basis. But MoveOn.org, the activist group protesting Beacon over privacy concerns, says it doesn't have to be this way. In fact, the organization told News.com, screenshots leaked prior to Beacon's launch indicate that a systemwide opt-out was once intended as an option for users. Facebook only later decided to remove this option, it seems. Here's the evidence.
After News.com's Caroline McCarthy posted this evidence, Facebook responded to say it's listening to feedback and will soon make changes. Should be easy. From the looks of the screenshot, Facebook just needs to revert to an earlier version.