WhatsApp also had no need for moderation or ads handling (IIRC, they used small yearly payments plus contracts with network operators).
Just handling the ads, both backend and user interfaces for buyers, would quickly net you several more developers.
It's easy to have only 45 engineers when you constrain the scope of what they have to do very well, but the scope was much, much, much smaller than Twitter's.
Yes, 55 employees iirc. I don't think it was 'surprisingly small', though. It's just that cash-rich startups have had the habit of hiring as much as they could instead of hiring the minimum required, which is actually the sensible way to manage a company.
I imagine the team responsible for maintaining the direct messaging feature of twitter is also around 50 or so people.
In case you haven't noticed, twitter has many, many, many other features beyond direct messaging, each of which requires its own team to maintain, and many of which are far more complex than a messaging client.
