If you see Apps (no matter if native or web) and websites like I do, you'll probably now the worst thing that can happen to an endless scrolling page: No shortcut to get back to top. Even Facebook doesn't offer a way to get back to top on their website nor do they on Facebook for Android.
On Android you can simply hit the menu button and the sidebar-like menu will fade in, so there is no problem with not having an "scroll to top" action because there is no need to get back to the top. I've nothing more to say on their app in this case (beside that it's the slowest Social Network App I can think of but that's another point).
On facebook.com you'll experience something that is, in my opinion, a bad UI/X idea: If you scroll down the sidebar on the left (that contains all the shortcuts to messages, groups and pages) will stay on top of the page while the Advertisement section on the right scrolls down - who thought of this? And what's the reason to fix the Advertisement while the menu, which is also important, isn't fixed?
I see a reason in fixing the Advertisement: People are more likely to read them and click them the longer they stay inside the viewpoint, but not fixing the menu is just a bad idea - there's no reason I could think of to explain why the menu should hide when scrolling without a possibility to bring it back. When you click the Facebook Logo on the top left the site will refresh and bring you back to top. In case of UX I'd rather like to have a smooth animation that scrolls the content back to top that having a completely refresh. What about people with slow Internet connection? They'll need to scroll up by themselves or have to wait until the whole site refreshed - this is, in every way, a bad example of UX. Even though Facebook has a lot of great minds onboard, some should maybe think twice about what to hide, what to fix and how to get back to the top.