For one thing, in 2009, WhatsApp co-founder Brian Acton, laid off from Yahoo, needed a new job. As shown in his tweets from that time, he was rebuffed by many, including Twitter -- and Facebook. We think things worked out better for him and fellow co-founder Jan Koum, a colleague from Yahoo.
It also turns out that Google offered $10 billion to WhatsApp, previously. Rebuffed, they were also rebuffed a second time: Google offered to pay the startup in exchange for the right to be notified if WhatsApp entered into acquisition talks with other companies. That didn't happen, either.
Finally, why would Facebook acquire a messaging app when it has Facebook Messenger, its own app. It's because Facebook wanted to gain more control over messaging in Europe and in emerging markets. Its there that WhatApp has a huge lead over Messenger.
That being said, Facebook has clarified things:
WhatsApp will complement our existing chat and messaging services to provide new tools for our community. Facebook Messenger is widely used for chatting with your Facebook friends, and WhatsApp for communicating with all of your contacts and small groups of people. Since WhatsApp and Messenger serve such different and important uses, we will continue investing in both and making them each great products for everyone.WhatsApp also confirmed the same thing: The company (WhatsApp) will function autonomously.