WhatsApp was founded by Jan Koum and Brain Acton. Koum attended Alex Fishman's regular meetings at his place. Local Russians often came to Alex for pizza and movies. Koum pitched WhatsApp to them. He hated it when he missed a call, so he wanted to make an app that would show people's statuses when they couldn't talk on the phone (e.g. "Low battery").
WhatsApp 1.0 had bugs and people didn't like it much. When Apple announced push notifications for iOS, Koum decided to launch WhatsApp 2.0, integrated with that feature. Now, people could get notified when their friends would change their status.
Koum and Acton also took advantage of an App Store's bug. The "What's New" list saw WhatsApp as a "new" app whenever the creators changed its name on AppStore. They kept changing it, and it gave them the first 1,000 users because WhatsApp stayed at the top of the "What's New" list until Apple fixed that bug.
Soon, people started using WhatsApp for instant messaging, and it sparked an increase in WhatsApp's popularity.