Facebook came across a bunch of duplicate SSL certificates for some of its own domains and revoked them immediately. Facebook’s own-developed Certificate Transparency monitoring service immediately detected those certificates. An improperly issued certificate could be used in man-in-the-middle (MITM) attacks to compromise encrypted HTTPS connections, putting millions of users’ privacy at risk. Google had launched ‘Certificate Transparency’ project in the year 2013, enabling anyone to detect easily fraudulent and stolen certificates. It does not come with in-built monitoring, and alert service does not automatically notify domain owners if any new certificate (legitimate/forged) has been issued for their domain.
Source: https://thehackernews.com/2016/04/certificate-transparency-monitoring.html