Yeah Facebook was (in)famous for autogenerated code in their apps and frameworks. In fact early on they had to do some hack on Android since they had more classes in their FB app than was supported by the operating system lol.
So super inefficient binaries, but I guess more efficient to develop (or I assume that was the idea)?
You are getting the details wrong (I was there). This was the single DEX limit, and Google would just bump it in AOSP every time their own apps hit it (as their apps didn't support older OS versions). FB at the time was supporting back to froyo, which had a much lower limit than "modern" apps needed. See this note for more info: https://www.facebook.com/notes/10158791578712200/
By the Dalvik Virtual Machine (DVM). 65k method limit is what Facebook hit. tbf, DVM was engineered to run on embedded systems with 64MiB RAM (as a replacement for J2ME).
Its Meta. They have enough money to spare not to need to worry too much about whether there is a good RoI on rewriting some mobile apps.
I very much doubt FB's mobile apps are written for any sort of efficiency, (either in engineering or financial terms) - 18,000 classes in the apple one! https://quellish.tumblr.com/post/126712999812/how-on-earth-t...