What do the various Facebook Messenger circles near messages mean?

Update January 2019: In the latest Facebook Messenger UI (user interface) update for mobile devices, the blue circles are replaced with gray circles. A 'Dark Mode' is promised as well, at some point in the future, and we'll have to wait and see if in that mode, these message status circles will be gray as well, or a different color.

For now, the web interface at https://www.facebook.com/messages or https://www.messenger.com/ keeps the same color scheme, with blue circles for messages statuses.

Original article follows.

Friends of mine ask me from time to time what the different circles in a Facebook Messenger conversation mean. Was the message sent? Did the recipient read it already? Here's a quick clarification on those markings:

Instead of the Download3k icon above, you'll see a small profile photo of your recipient

Facebook's Messenger uses 4 types of icons for different stages your message is in:

1. An empty blue circle

At this point your message is being sent. It leaves your phone and goes to Facebook's servers, but has not reached Facebook's servers yet. In order to be sent, you need a working internet connection on your phone. If your phone is struggling to connect to the internet at this time, the empty blue circle will stay until you do have internet connection.

If, after a while, you see a red exclamation triangle and the message "This message didn’t send. Check your internet connection and click to try again." it means sending failed and you'll need to retry.

2. A blue circle with a check mark

At this point your message has been sent and has reached Facebook's servers. Facebook now tries to deliver it to your recipient. If they don't have an internet connection (poor GSM connectivity for example, no GSM signal area, etc.), the message won't be delivered yet and it will stay marked with a blue circle with a check mark until your recipient's phone gets an internet connection.

In case your recipient has blocked you, the message will stay marked like this.

If your recipient is not yet in your list of friends, they will first receive a Message Request, which they can Accept or Ignore. Until they accept it, the message status icon will remain a blue circle with a check mark.

3. A solid blue circle with a check mark

Message has been delivered to your recipient. More technically speaking, the Messenger app of your friend has downloaded the message from Facebook's servers.

4. A small profile picture of the person you are talking to

The recipient of the message saw and read your message.

Now you can only hope they'll write back ...