> I do find it curious that WhatsApp was operating at a much larger scale in terms of users and messages processed when they were sold to Facebook

I recommend you search around HN before you make sweeping generalizations, there are numerous threads that explain how or why WhatsApp were capable of doing what they were. From a technical perspective, as a "core product", Twitter's business has more abstractions

To get you started[0]

[0]: https://news.ycombinator.com/item?id=10225096