(510000 + 293000 + 136000) / 60 is about 15.5k, so slightly less than the estimate in this article, although you are right that I was overstating the amount of traffic that is read only.
Yes, but that's not counting things like post likes, FB Messenger, check-ins, account creations, profile edits, logging and statistics, etc etc. I wouldn't be surprised if they're in the millions of writes per second.
In a recent submission [0] they give some stats about their abuse system and at ~47:35 they mention this system gets 1M requests per seconds, which doesn't have to precisely coincide with write volume but suggests it's in the right ballpark.
