I actually agree with you that this is "drug dealer" tactics but for totally different reasons. I think this is a classic "the first hit is free" move.

It's a long-term play. I believe FB when they say they won't show ads to kids, or use their chats for ad scraping. What they really want is for those kids to become so familiar and dependent on the system (FB), that they naturally transition to being active FB users as adults.

It's not a secret that FB has a perceived "uncool with kids" problem - hence the rise of things like Snapchat. If you hook them when they're really young, though, then FB just becomes an unquestioned part of life.

As far as the tool itself, i dunno - I mean messenger tools have been a fact of life for quite some time now. AIM, ICQ, etc. I don't think FB messenger is particularly different other than the fact that it's attached to, well, FB.