Message Lib instead of Mail only ?

Can you see the problems with this when Facebook, Twitter or Google updates their API's?

As I said Message lib with e-mail supported "gateway" by CI4 in default.
All 3rd parties code should be out of the ci 4 source . 

But in your code it would be a lot easier if you got..

$mailMessanger (instance of Messanger Mail) ->sendMessage($from, $to, $message...); .. 
$fbMessanger( instance of 3rd party Messanger) ->sendMessage($to, $message .. - $from should be the app user or what ever is implement)

At the end Mail, FB notification, message and etc.. all of those things are messages From(user/app) To(user/app)..

P.S. Such message system can be used at the same time for external and internal messages depending on which "driver" its using and how its configured..
(if you need to send either internal message or/and an email to some client) ..

Best VPS Hosting : Digital Ocean