I think FB had to acquire one of these messaging apps, because their own app was not gaining traction, especially outside the US. Growth in use of messaging apps is exploding, because of continued growth of smartphones, and because mobile carriers got greedy and charged too much for text messages (SMSes) even though it basically cost them nothing. (SMSes use the control channel of the mobile network.. basically riding on existing bandwidth.)
In the end, communication is a key human need, by various methods, and so I don't see the ability to communicate in text format with your friends going out of style any time soon. Sure, the apps used to do it may change, but not the way it's done.
So FB had just a few companies to choose from with a big userbase, particularly abroad, like Viber, Line, WeChat, etc. They probably made some offers to some of those companies, too. The best fit from a company culture and logistics standpoint was WhatsApp -- started by Silicon Valley people, located nearby, funded by VCs they know, etc.
FB's
balance sheet is not bad -- about $13 billion in current assets (cash and similar) as of the end of 2013. This acqusition was mostly in stock, so there's not a big cost to operational risk by doing it.
I'm not sure I'd do the same deal if I was Zuck, but I do see
why they did it.