Friends tell me that very sensitive code (ranking algos, etc) is not in the repo.

Personally, I also favor a single repo. You manage it the way you manage separate packages: with organization and some discipline. The magic is that command line tools like grep, sed & awk--and static analysis tools for refactoring--work really well. You can change a method signature and it just works. I've been part of monolith-breaking before (most recently at Trulia) and it definitely adds friction to the development process to work across such a rich graph of package dependencies.