I suspect a lot of people use Facebook because they've always used Facebook - mostly older folks at this point - and a lot of the younger folks go there because their favorite association / football club / local restaurant / library are there too.

Meaning Facebook's success today is mostly inertia: there's no way any open-source replacement would pull a critical-enough mass of people away from Facebook to fight the inertia and enjoy any meaningful success. I'm pretty sure open-source developers know this and chose something fancier and more exciting to spend their time on.

Creating a Facebook replacement would be about as exciting as creating a Visual Basic replacement: millions of people still use VB, but nobody wants to touch a dying technology.