Facebook spent billions pickup up Whats-app and Instagram (and probably a fair chunk developing and maintaining messenger).
This wasn't solely to pick up untold millions of new users who'd never used "original blue facebook" - if it had been, you'd need a facebook account to use Whats-app. Point was to ensure you didn't stray from the blue-app, onto something Facebook didn't own/control.
In my humble view, the keeping of messenger and whats-app separate by Facebook is smart - as they have two offerings that appeal to different people (and to many people that happily use both).
You know from all the rumours of "ads on Whatsapp" that internally in facebook they're trying to work out how to leverage all our eyeballs on all those different apps - but the reason they haven't merged them all, is they've worked out they keep more people if they keep the apps separate.
To take my favourite anecdotal example, I used to use latitude on google maps to work out where my friends were (I'm in the pub, when are you turning up?). Then google killed that functionality to try to make us all move to google+.... We didn't, it died.
Now, if a few of us are trying to co-ordinate say a drink after work, we'll use whatsapp. Google lost that feature I use to Facebook.