Respectfully, I don't know if that's the right assumption. Facebook has users. A metric shit-ton of them. :) In fact, many of WhatsApp's users are Facebook users, and vice versa.

This isn't a user acquisition play. It's a user engagement play. Facebook -- like everyone else in the world -- is well aware that it has an engagement problem. It also has a retention problem, especially with younger users (who are flocking to messenger programs). Finally, Facebook has some challenges internationally, where the social networking space is more regionally fragmented. WhatsApp is a lot bigger overseas than it is in the US (largely because it's a free alternative to SMS in countries where SMS carries significant charges).

So what's this really about? This is another defensive acquisition on Facebook's part. Entirely overpriced, in my humble opinion. Instagram was, in retrospect, an incredibly smart defensive buy. WhatsApp at $16 billion? Less clear.