Server down cases

I am not sure whether I am asking this question to the right place however,

A general question,some days earlier I have heard that facebook server has gone down for 15 mins.Some years ago i have heard the same news in some other big company that server has down for 20-25 mins.They have a huge datacentres with DR's available also.So, How their internet sites went down.

Kindly assist its reason.