Facebook and Google are not like your regular website. Why Facebook and Google never go down for maintenance?

Their services operate behind load balancers.

When a server or even an entire data centre needs to be serviced, the load balancers simply send the traffic to other servers.

Also, these websites settled up for scaling and deployment without any downtime.

They do the maintenance, deployment, and scaling without causing any downtime.

Applications like Facebook have multiple servers working together.

If one of the servers/or any of the server needs maintenance or goes down, another server will take its place.

However, even with all this, Facebook has suffered with multiple times downtime in the past.