Idle is a status in Facebook chat designed to let your friends know that you are available but not currently paying attention to your Facebook screen. If a friend sends you a message while you are idle, he knows you might respond to it, but are unlikely to do so immediately. Facebook uses two factors to determine how you become idle: if you are inactive and for how long.
You are unable to set yourself as idle on Facebook chat -- is a status automatically assigned to you by Facebook. While you cannot force yourself to become idle, if you don't want friends to see you as available it is possible to go offline in chat. Just click on the "Chat" icon in the bottom right corner of your Facebook screen and then the circular wheel icon at the bottom of the chat sidebar. Click on "Available to Chat" to remove the check mark next to the option and become unavailable.
Your friends who are logged into the chat service will see you as idle if you have been inactive on Facebook. Activity might consist of engaging in a chat conversation, but activity also includes browsing other users' profiles, commenting on walls and any other action you take on the site. If you fail to be involved in any activity, Facebook automatically lets your chat friends know you are idle.
You are only made idle on Facebook when you have been inactive for at least 10 consecutive minutes. As soon as you begin to browse on the site again, or take part in a chat conversation, you go back to available status until you once again stop using the site for at least 10 minutes.
Facebook friends are made aware of your current status via symbols next to your name. If you have a green circle, you are currently active and available for chat. Idle users have a blue half moon next to their names. If you are unavailable altogether, there is no symbol next to your name when friends view your name in the chat application.