Again, it starts with a customer list that a business has already created — for example if I’ve given my email address to the bookstore on my block so that I can hear about future sales and events. Businesses will be able to upload those lists of email addresses, phone numbers, and user IDs to Facebook, though the data will be hashed first so that Facebook doesn’t have access to that information. Meanwhile, Facebook’s user data will be similarly hashed, so the company can compare both sets of hashed data, creating a list of users whose contact information matches up with what the advertiser uploaded.