Just the number of active users is a really bad metric in this case. WhatsApps product is much, much simpler than twitters products.
Can you enumerate Twitter's products?
Perhaps Instagram is a closer comparison -- broadcast photos instead of broadcast 140 characters. And you have a much smaller engineering team, much faster growth, twice as many active users, and far fewer fail whales.
Regardless of the comparisons, 2,000 engineers seems excessive in proportion to Twitter's product, which suggests engineering inefficiency.
Instagram has only recently started to monetize the product -- it's almost unfair to compare the size of the engineering teams supporting the systems.
Also, I suspect that their strategic plan is not about minimising the number of engineers. They may want to be able to set up non-public projects which iterate for years or months before becoming visible: frameworks, app ideas, you name it. Their Fabric platform gives them excellent insights into what people are doing on mobile apps, for example. The kind of insights and understanding they have into personal and aggregate app usage is quite frightening when you realise just how much they have instrumented.