The half moon symbol is part of Facebook chat, a realtime chat program that automatically activates whenever you’re on Facebook. You can use Facebook Chat to chat with your friends in realtime. The moon, when shown beside the name of a contact, indicates that the person is “idle,” suggesting that they have not been recently active on Facebook.
Not to be confused with your Facebook status posts, status icons serve multiple purposes in chat programs. Most often, they signal whether or not a person is available to chat. Facebook does not have the same variety of status icons as other chat programs: You are either available, offline or idle. However, you cannot simply set an “away” status.
When viewing your friends list on Facebook chat, you will see either a green dot or a half moon beside each name. The green dot means the friend is available, while the half moon means the friend is idle. While idle friends may still be at the computer, they are not currently looking at Facebook, so they may not respond to your message immediately.
Can I Set My Status to Idle?
You can’t manually set your status in Facebook Chat. However, you can choose to go offline or online by clicking the “Options” button in the chat window, then selecting “Go Offline.” You’ll automatically become Idle after 10 minutes of inactivity on Facebook.
What to Do
If a friend goes idle, you may still be able to reach him — just message him and see if he responds. Alternatively, you may have to make contact via other methods such as cellphone or text message. If you’re waiting for a contact and want to avoid being shown as idle, continue to navigate Facebook every few minutes.