Airbnb uses a monorepo for JVM-based project but most of Airbnb's code at least as of mid-2017 was not run on the JVM and was hosted multi-repo.
Uber has navigation, route optimization, queuing, etc. Facebook has to propagate activity our to massive and complex social network graphs.
I'm not discounting the toughness of operating at Airbnb's scale, but from my limited understanding it seems like they are not solving a new problem.