So, to clear the Facebook Messenger notification: Open a browser on your desktop (not on mobile) Navigate to Facebook.com and log in to your account In the left navigation, click the "Messenger" link You should see the unread message in the left indicated with the text in bold font Click the message and Facebook Messenger will mark it as read. Facebook Messenger Icon: Red Triangle + Exclamation A red triangle with an exclamation point should only appear if your message is not sent due to an internet connection outage. A message should appear alongside the red triangle that says "This message didn't send. Check your internet connection and click to try again.".