Because it is actually more than just one program.
I'm pretty sure it is almost like trying to sift through the entirety of, say, the Linux project's source code (largest open source project) or the KDE project's source code (second largest). One is the base of an operating system and the other is a rather popular desktop environment for both Linux and Windows, that also happens to come with several useful programs and a cross-platform development platform for the desktop.Facebook happens to have quite a few services, and it not only has a rather large front end (if it consists of less than 100,000 lines of code, I'd be surprised) but a massive back end with a database. It also stores a lot of information and files for literally hundreds of millions of people.
Facebook also has its own open source libraries it internally uses (like React.js), and two different development platforms, one for mobile and one for the browser.
This should probably help a bit in understanding why it is possible.
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