[aeyes]

Facebook is primarily a message and photo sharing application but it also has group boards, dating, live video streaming, a marketplace, browser games and probably more features. Then there are all the features for targeted ads, tracking, payment and so on. All of this is available almost all around the world.

How do you implement these features for more than a billion active users in less than a million lines of code? Even 100 million sounds like a low estimate.

Why does the Linux kernel have more than 30 million lines of code?

You could ask if these features make sense. But in the end it is their business decision to make.

[gpderetta]

how does the number of active users affect the size of the application?

[isbvhodnvemrwvn]

You need to support a huge range of devices and cultures.

[aeyes]

Aside from the technical challenges of running a system with such a massive amount of data:

translations (left to right text and layout: https://sy-sy.facebook.com/), different laws depending on jurisdiction; for example privacy/moderation/takedowns, taxes/payment/invoices if you are selling anything, accessibility

If you ever start a company and you have the long-term goal to offer your service in multiple countries I would advise you to launch in at least 2 countries, preferably supporting different languages.