The billionaire said sorry after an internal technical issue took Facebook, Messenger, Whatsapp and Instagram offline at about 16:00 GMT on Monday.
The scramble to bring it back online eventually succeeded at around 22:00.
But it is likely to increase scrutiny of the social media giant's reach.
For hours, potentially billions of people found themselves without the social media tools they relied upon to keep in touch with friends and family. Others reportedly found they could not access services which required a Facebook login.