Facebook has several engineers working around the clock 24 hours per day 7 days per week and even on Holidays to ensure the data center stays up and operating so that Facebook and it's servers can be accessed without issue and to prevent downtime.
Well they really are not engineers but instead data center techs and people working in the Network Operations Center as well as in the data center server room itself.
The amount of techs that Facebook has for each data center is not really known for sure but they probably have 100 or more techs at each data center ensuring the servers are operating and the network is online and accessible to everyone around the world.