Reading between the lines it seems the problem wasn't scaling, but programmer productivity. Smaller code bases is often easier to work with so I guess they solved that by dividing it up into many small services. The blog could use a more detailed description of the problem they are actually solving.
But I finally realized - those messages are probably mostly tracking, ads, more tracking, some infrastructure work and even more ads & tracking. The sausage machine that turns people into money.
> 100 million reviews/year is only 3 reviews per second on average