Or, facebook(https://www.facebook.com/business/learn/facebook-ads-reach-existing-customers https://www.facebook.com/business/learn/facebook-ads-reach-e...) matc

Or, facebook(https://www.facebook.com/business/learn/facebook-ads-reach-existing-customers https://www.facebook.com/business/learn/facebook-ads-reach-e...) matches this cookie to your fb account in their system. Then making this information available across the browsers is trivial. Now, to actually think of implementation, I can't even imagine the amount of data they need to store(and clear in LRU manner) to make this work.