It's a bold assumption that those open source projects are larger than facebook, a company which employs 2000+ engineers.

webkit and chromium have about 6 or 7 million lines of code and < 200,000 commits.

Facebook 6 months ago had 62 million lines of code and > 1000 commits a day.

http://www.quora.com/Facebook-Engineering/How-many-lines-of-...

Also, there's a difference between those open source projects using large repos with git and Facebook wanting to increase developer efficiency. Developers sitting around waiting for a rebase doesn't really pay off, where as an open source project can get away with it.