The colored circles on Match tell you the activity status of a user. For example, a solid green circle next to a user's name means that they have been online within the last 45 minutes. If the green circle is empty, it indicates that they were last online between 46 minutes and 24 hours ago. An empty yellow circle means the user was last active between 24 and 72 hours ago. Lastly, no circle signifies that the user has not logged in for over 72 hours.