This is how it works
Similar to common push notifications, the user gets an Opt-In query. He also can sign up for facebook notifications via hook. Those can pop up now in the messenger app on the smartphone, on the actual website of facebook.com or on messenger.com.
Accessible always and everywhere
Most aktive facebook users are usually easy to reach via messenger. That means high traffic and the notifications invite for instant interactions.
The alternative to iOS
At the moment, apple sadly doesn’t allow push notifications on websites, only in the apps themselves. Which makes facebook messenger a true alternative until that changes.
Action - Reaction
Especially for sites with lots of traffic from the internal facebook browser, messenger opt-Ins are quite the attractive opportunity. Here, the user is directly logged into facebook and only has to push one button to sign up for messenger notifications.