Social networking is all about network effects. The biggest threat to Facebook's position at the top of the pile is that another social network will come along, start to gain traction (most likely initially with users that aren't current Facebook users - e.g. early teens) until it reaches a tipping point where current Facebook users start migrating to the new social network because that's where the people they want to interact with are hanging out.
Once that tipping point is reached, the migration is likely to happen rapidly and irreversibly. By acquiring companies like Instagram and Whatsapp, Facebook is not only preventing them from usurping its social networking crown but bolstering its own position against future emerging competitors.
$19bn might sound like a lot but it's cheap when you consider that the risk of not buying Whatsapp is that it eventually obliterates Facebook in the same way that Facebook obliterated Myspace and the original Bebo.