> not tested 10 trillion times a day

Perhaps not, but it might be close: essentially every message sent between every component of every service on every machine in every datacenter Google operates gets encoded as a protobuf. Engineering at Google consists of populating protobufs to send to other services, which reply with protobufs containing the data you need to stuff into more protobufs, which you can then send on to further different services.

We can, in other words, safely conclude that protobuf scales.