Visa has a $80 billion market cap, $9b in sales, and $3.6b in profit with a mere 7,500 employees. One of the best ratios of any major company. In fact, it's better than Facebook's current employee to sales ratio (and it's worth pointing out that they're nearly at a $10b sales run rate, and $4b in annual profit).
Visa has around a 40% net margin, that's beyond extraordinary. There's almost zero chance Facebook can be any less bloated than what Visa is already doing.
Discover also has a stellar ratio, 11k employees to $8.5 billion in sales.
Ditto MasterCard, with 6,700 employees and $6.7b in sales.