True, the reasons are different. But the motivation is the same : to stay ahead of the competition. In my view, and based on my own experience, Whatsapp is being used as a light-weight social network built around messaging and optimized for mobile phones. In fact, I share a lot more stuff with friends and family on Whatsapp - including pictures/videos/messages - than I do on Facebook.
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.
But it in no way ensures they stay relevant. It is literally just blindly flailing around buying anything that gets a lot of users out of fear and having no idea how to maintain their position.
>It doesn't ensure they stay relevant but it's a better strategy than watching your target demographic leave your platform.
I don't see any evidence to support that claim. It is a different strategy, but I don't see any indication that it is better. Flushing a few billion dollars down the toilet is a different strategy too, but that doesn't make it better.
Facebook can mine some more data from this I guess, but something rather marginal from what they already have.