Founders sell WhatsApp to Facebook: Motives not clear

The founders of Whatsapp were very clear that did not not what to sell advertisement through the messaging app.  (See their 2012 statement.They did not even want to collect data on their users.  So why did they sell themselves to Facebook, which is all about collecting more data on us to sell it to advertisers?  Here is a clue in their blog.  Maybe they were tired of having to manage a business rather than just design a product.