The Reddit front-end is a relatively simple application. It has a page with a list of stories. It has a page with one story and a bunch of comments. It has a page to add comments. It has an endpoint to vote up or down stories, and to vote up or down comments. And maybe a few other less-interesting things, like admin interfaces.
Facebook is surprisingly easy to underestimate even as someone using it a fair bit.
Just try find every single interface in the front-end. For your own profile. Feed is front-and-center, as is timeline. Then look at events, groups, &c.. Look at messaging.
Then look at the interfaces for managing your privacy and permissions. The apps that you're using and information about when last they requested information. Security like login approvals. Then think about the flows involved in reporting content as abusive or inappropriate. For verifying your identity if you've forgotten your password. For adding more security if you log in from a new computer or from a new location.
Then look at pages, including insights and scheduled posts and so forth. Then look at advertising - boosting individual posts, creating campaigns, &c.. Then look at the interfaces for developers. For translators. Interest lists.
The backends for most of this are in C++ and Java. There's a large amount of data processing happening to track hidden things like spam and scam prevention. But the front-end surface area is quite clearly an order of magnitude or maybe two larger than Reddit (at least where it was when this happened).