The fact that they notified you means it wasn't exactly silent. I saw this notification and turned it off. I wish it hadn't been opt-out and had been an opt-in thing instead.
It's silent unless there was a proactive message to you from Facebook that your face will start to be recognized. This facial recognition happens even when you're not using the app.
So Facebook is waiting for you to take action before telling you anything. That is not proactive, it is silent.
What if you don't go on Facebook any more but have an account? You'd never know this was happening.
"I didnt get your email. I have not logged into my account lately."
It seems like semantics. You expect facebook to send an EXTERNAL notification to an operating system like ios/android or to an email provider like gmail/hotmail. Other people consider it not silent for facebook to make sure it shows up in their newsfeed.