I know nothing of the code Facebook is hosting, but one fairly obvious general benefit of having things in a single repository is that you can use relative paths to link dependent projects, like if you have a directory structure such as:

libs

libs/mylib

apps

apps/myapp

then myapp can reference library mylib via ../../libs/mylib

If all of these things are in separate repositories, all bets are off about how their paths relate to each other on the local filesystem, resulting in more complicated build procedures.