Facebook is testing and will soon launch Facebook Exchange, a real-time bidding ad system where visitors to third-party websites are marked with a cookie, and can then be shown real-time bid ads related to their web browsing when they return to Facebook. This could be a huge money maker for Facebook as it will allow for better direct advertising — for example a travel site could serve ads about a flight to Hawaii to someone who almost bought a flight on their site. Advertiser might pay big premiums for highly-accurate targeting.

Users will be able to opt out of Facebook Exchange, but only on an advertiser-by-advertiser basis. They can’t opt out of the program completely. Facebook just notified TechCrunch that Exchange is currently in testing with roughly 10 advertising demand-side platform, and it will become more widely available in the next few weeks.

We’ll have more details shortly.