FB and WhatsApp are once-in-a-decade type returns. In baseball parlance perhaps a World Series winning grand slam or a perfect game. You needn’t exit for $19B for it to be a great outcome.
The top line acquisition price is probably the full package including costs and retention bonuses, not all of which will be paid out. The only people who will get something for their time are the founders, but they're probably looking at low to mid 8 figures at best.