WhatsApp before acquisition had 55 employees and was well known.
And most companies making billions need thousands of employees, especially software devs need to understand that.
And so they outsourced everything to others. Their 2013 expense is $148M for $10M of revenue[1]. They aren't spending $148M on 55 employees for sure. And that doesn't include server cost(which is likely very low due to optimizations).
[1]: https://www.sec.gov/Archives/edgar/data/1326801/000132680114...