I've always considered it as a defensive play rather than a positive one - Facebook have this beautifully well-entrenched network effect that defends their position - their unique selling point is that they know the world's social graph, and it's terribly difficult for anybody else to try to build that data as long as Facebook provides an acceptable alternative to any new communication medium, because it's always going to be easier to engage with your friends on a platform they already use.
In WhatsApp's case, though, Facebook screwed up - they didn't offer a mobile messaging service that was convincing enough to take the easy victory that their position should have given them in that market, and that chink in their armour let WhatsApp build a social graph and a toehold in the space that could have made them a true competitor, so Facebook paid to make the mistake go away.
Flippantly repeating a HN mantra: who cares about making your life easier, you're the product, not the customer.
More seriously though, the value of knowing the world's social graph is certainly more complicated than I feel qualified to talk about, but it's pretty evident that it has value to advertisers, which is really what's important.
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.
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.
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.
