Pre-acquisition WhatsApp handled 450 million users with 35 engineers paid at Asian tech salaries, or roughly USD$ 30-50k. So WhatsApp's entire engineering salary cost is equal to what Signals pays just 3 of its engineers for 9x the userbase. WhatsApp had roughly 257 users per engineering dollar spent, while Signal has
at best 20 users per engineering dollar (but that number is lower because I'm only accounting for the "highly compensated" engineers and not Signal's full engineering staff).
Instagram had 13 employees with 31 million users when it was acquired by Facebook.
I don't know what the post-layoff employee account is for Meta Messenger as they did not disclose it in their SEC filings, but pre-layoffs it was several hundred.