[chrisguitarguy]

I suspect it's because WhatsApp's backend can be turned into a platform that will run WhatsApp itself as well as Facebook Messenger.

There's a lot more opportunity to do messaging between businesses and individuals (eg. customer service) as well -- see messenger for business.

Not saying Facebook isn't out to sell ads, just that there's probably a lot of reasons that Facebook might want WhatsApp's technology and team. Especially as they try to move more conversations onto their platform and reach out into other markets.

[netcan]

I imagine the biggest driver was what people postulated at the time.

It's the messaging app that the most people are using. It's used for updating groups of friends, sharing photos and all the other stuff that Facebook needs to be The Place for. It also has a simple, organic-ish mole for deciding who sees what and when that intuitively makes sense to people. IE, you don't have to twiddle with circle settings to find the nexus of friend-work and professional-friend that should see your message. Instead it just has groups that start out as "who wants to watch the fight at the bar" and sometimes people keep posting to them.

Whatsapp could have pivoted or been acquired for directly competing with Facebook.

[alandarev]

No company was acquired for its "backend".

[otterley]

Nonsense. Google's acquisition of Applied Semantics (which developed AdSense) became a major revenue engine for the company. There are countless other examples as well.