Thank you for saying this. I was going to say the same thing. The difference between facebook and my side project is
not the core functionality. Anyone competent can build something that seems to look and act like facebook in a weekend or two in the general sense of "look and feel." Which is also perhaps a dangerous way to go, lest you get sued.
It's getting the nitty-gritty details hammered out and then implemented that takes time and people. I'm currently trying to do something similar but directed in a very different way. Facebook feels like a CRUD app. But there's a lot of different moving parts that you have to deal with on top of the CRUD pieces.
And every single one of those things--while not necessarily difficult to implement individually--add up to tons of decision time.
When the expected outcome is already decided, the problem becomes much much easier to solve for a developer.
Assuming that you're dealing with some kind of a reasonable web stack, implementing any individual feature is often not that big a deal. Deciding to do it, and also perhaps making the choice to have a reasonable web stack to begin with, are potentially problematic.
And of course, building something that can massively scale is hard as well.
But from my experience with side projects and also my day job, the hard part is making decisions.