Give a store your e-mail address, it'll find you on Facebook
arstechnica.com/business/2012/10/give-a-store-your-e-mail-address-itll-find-you-on-facebook
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
In the post written by Facebook privacy engineer Joey Tyson, Facebook highlights a new use for those e-mail addresses you've been tossing off while checking out at your favorite stores: a store can hash the e-mails it has in its database, Facebook will hash its own, and then the two parties will compare notes to find customers to show that store's ads to. No actual information is shared, so Facebook skirts its third-party sharing promise again. Still, we don't expect most Facebook users who have given out e-mail addresses to retail stores expected them to be used in this way, hashed or not.