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.

[0] https://news.ycombinator.com/item?id=14624802

[1] http://events.techcast.com/bigtechday10/Garmisch-1345/