Your customer list is hashed locally on your browser and then sent to Facebook. There, it's matched against our existing list of our users' hashed IDs and the matches are added to your Custom Audience.

Hashing turns the data in your customer list into short fingerprints that can't be reversed. It happens before your data is sent to Facebook. Here's how it works:

1. Facebook hashes the data that our users have provided us. You begin uploading your customer list and it's hashed locally in your browser before it's uploaded to Facebook.
2. After your hashed data is uploaded to Facebook, we match it against ours.
3. The matches are added to a Custom Audience for you.
4. The matched and unmatched hashes are deleted.

* Nguồn: Facebook