Check marks on Messenger are an indicator that your message has been sent, delivered, or seen. When a blue circle with a checkmark appears next to your message, it means your message has been sent successfully. If the circle filled with a blue color indicates that your message has been delivered. Moreover, a small receiver's profile icon will show next to the message when someone sees the message. Apart from all this, if it is a circle only without a check mark, indicates that your message is still sending or has a slow internet connection.