The more Whatsapp satisifies my sharing needs, the less I use Facebook. And that is a competitive threat to Facebook. Instagram was similar - a light-weight social network built around photo-sharing.
I think Zuckerberg is a deeply strategic thinker trying to avoid the pitfalls of the "Innovator's Dilemma". Both Instagram and Whatsapp are classic examples of what is described in Innovator's Dilemma - light-weight products that can slowly eat into the market share of the incumbent, and ultimately displace it.
Edits : formatting.