From the press release[0]:

  "... and WhatsApp’s core messaging product and Facebook’s existing   Messenger app will continue to operate as standalone applications."

Why is that, I wonder. I was really excited when I heard the news since WhatsApp is the most used app and Facebook Messenger is the second, they becoming one would have been nice.

And chatting my friends on WhatsApp using Facebook Messenger [on the web] would be a killer feature for me, I feel the need for a web interface/desktop app every time I receive a message on WhatsApp and my phone is away.

[0] http://newsroom.fb.com/News/805/Facebook-to-Acquire-WhatsApp