A client of yours would like to target their customer database.

Custom Audiences were designed to maximize information security and privacy. Facebook systems have been audited by an independent third party, PricewaterhouseCoopers. They confirmed that Facebook’s secure mechanisms protect advertisers' information throughout the creation, use, and storage of their Custom Audiences.

Hashing is a process that turns information you share with us into short fingerprints that are impossible to reverse . 

Here's how it works:

When you begin the process to share your customer list, it's hashed locally in your browser before it's uploaded to Facebook.

We match the fingerprints of your hashed list against ours.

The matches are added to a Custom Audience for you.

The matched and unmatched hashes are deleted.