An important point to add to these observations is that WhatsApp made massive inroads with carriers in developing countries to zero-rate their services (unlimited WhatsApp messages as part of a data plan). This is the underreported aspect of the FB acquisition as these arrangements were immediately parlayed into zero-rating for WhatsApp and Facebook. WhatsApp was so far ahead of Facebook in becoming established in these markets that it posed a real threat. And as you said, $1-24 per year paid to WhatsApp would be only a fraction of what even the poorest mobile phone users pay for their plans.