Facebook commonly runs A/B testing on their UI. It is almost weekly for me and one of my friends to ask each other “hey do you have the <x> tab at the bottom” for Meta apps. Marketplace, Dating, “All Chats” in messenger which was just the same as the slide out menu I bet people didn’t use much. I also think they change per-user depending on what they use.

edit: I decided to check real quick and I do have the friends tab. Here’s a crop of it, note I edited out the last “Menu” tab for privacy.

https://imgur.com/a/6pFa1XF

Tabs are: Home, Friends, Marketplace, Dating, Notifications, Menu.

Mine has Home, Friends, Video, Marketplace, Notifications, Menu. They adjust whether to show dating based on relationship status...

Not only was that Friends tab not there for me by default, but it also does not do the aforementioned when I customize the top(? not bottom) tab bar to I include it. What it does is to show me a list: of pending friends, and friend requests. No space to show any posts to begin with. To see my friends' posts, I have to click the hamburger, then Feeds, then Friends, then (sometimes) manually pull down to refresh, because it usually just lies to me that I've already caught up. This is designed to be actively user-hostile, as if they were forced to implement this against their will.