Jan Koum actually stated at Startup School that whatsapp started monetizing to REDUCE load because they were acquiring users too fast and that it backfired, ultimately leading to profitability. From a strategic standpoint, whatsapp puts facebook in a great mobile position in a lot of emerging markets outside the US.
Part of that may be due to the fact that WhatsApp grows so fast that most users haven't been around for a year, but I'm under the impression that after a year, if you don't pay, the app continues to work and you never lose access.