The WhatsApp Business API is launching to let businesses respond to messages from users for free for up to 24 hours, but will charge them a fixed rate by country per message sent after that. Only charging for slow replies after 24 hours since a user’s last message is a genius way to create a growth feedback loop. If users get quick answers via WhatsApp, they’ll prefer it to other channels. Once businesses and their customers get addicted to it, WhatsApp could eventually charge for all replies or any that exceed a volume threshold, or cut down the free window.