Here’s Facebook’s full statement, via a spokesperson:
Last month we stopped offering email password verification as an option for people verifying their account when signing up for Facebook for the first time. When we looked into the steps people were going through to verify their accounts we found that in some cases people’s email contacts were also unintentionally uploaded to Facebook when they created their account. We estimate that up to 1.5 million people’s email contacts may have been uploaded. These contacts were not shared with anyone and we’re deleting them. We’ve fixed the underlying issue and are notifying people whose contacts were imported. People can also review and manage the contacts they share with Facebook in their settings.