You can access the source code of Facebook just like any other website using your web browser. Keyboard Shortcut (Windows/Linux) or Command + Option + U (Mac) while on a Facebook page. Right-Click Menu
Most people who venture into the source code of Web Facebook are there for a specific, almost archeological purpose. They are searching for the invisible. view sourcehttpsweb facebook
Here is a long-form, in-depth article on that topic. You can access the source code of Facebook