Facebook uses several simple symbols to identify which of your messages are new and which friends are online to chat. These symbols include colored closed dots, open dots and crescent shapes. To check which of your messages are new, look for a blue dot.
Blue Closed Dot
Click on "Messages" under your profile picture on your Facebook home page. This brings up the first page of your most recent messages. Any unread messages have a filled-in, or closed, blue dot to the far right of the page, in between the date received and the "X" you use to delete the message.
Blue Open Dot
If the message has been opened, the dot to the right of the message is an open, or unfilled, blue dot.
When you hold your mouse over the closed blue dot, you have the option to mark the message as read. Click on the dot to change it to an open blue dot. Click on the open blue dot to mark it as read, or change it to a closed blue dot.
Facebook uses other symbols in its chat feature. Under your chat list, friends who are online and available for chat have a closed green dot to the right of their names. Those who are idle, or who haven't been active in at least 10 minutes, have a blue crescent shape next to their names.