Literally, Facebook reverse engineered Android's vm solution to monkey patch in a longer method table instead of reducing the method count. I have no idea why but their massive amount of client code seems very important to them.
Part of it is FB’s company culture — generally the ethos has been to enable engineers, technical precedent be damned, and to never say no to a product idea. It is really impressive and a major part of why FB can move so fast despite being a very large corporation.