It's plausible. They crossed $20BB valuation roughly at the end of 2009. At that time they had 400MM users. This is about the same growth rate as WhatsApp since founding.

However, Facebook had 400MM largely North American and European users and had $1-1.5BB revenue with obvious potential for much more. WhatsApp's revenue was a couple orders of magnitude less after five years, with lower potential on its own to grow since it lacked Facebook's ability to collect and exploit data.

So, it seems like WhatsApp should be valued less than Facebook, i.e. less than $20BB, at the same stage in its life. That doesn't mean Facebook overpaid; WhatsApp's users were nearly a perfect complement to Facebook's geographically and Facebook, unlike a company like Google, had the organizational ability to allow WhatsApp to grow while benefitting from Facebook resources.

https://dealbook.nytimes.com/2012/02/01/tracking-facebooks-v...