Whatsapp was having comparable traffic and users with something like 20 engineers. It is not directly comparable, as all whatsapp users at the time were app users whereas twitter's users are web users (more expensive to run). Furthermore, with everything public, Twitter needs a lot more policing than whatsapp ever did. But they can probably reduce their stuff to 2-5 percent of current size and still provide the same service that users get - though it will have to be better engineered than what they are currently running.