it's not just facebook... they have oculus, insta, whatsapp, they were exploring mapping, and self driving cars at one point...
Many interviews casually ask you to implement a chatting app. By yourself.
But I agree that Facebook is significantly larger in scope.
For example, they didn't store contacts/images/messages server-side as you see in Telegram, Google ChatAppOfTheWeek, FB messenger, Twitter, IG, etc. All the infrastructure and the folks required to develop and maintain it, simply didn't exist. Similar with the limited amount of data collection they did at the time -- if you don't log it you there is no reason to have a team of people to analyze it. If you don't have ads you don't need an ad sales team. Etc.