Scrolling Through Your Facebook Chat Contacts
To navigate through your contacts list for someone to chat with, take your finger and swipe downward (and vice versa) to view all your available online contacts. To select a contact to chat with, click their name to continue.
An Explanation of Facebook Chat Contacts Icons
The majority of your contacts will have a green dot appear next to their name. This signifies the contact is actively using Facebook, whether via computer or using a mobile device.
Contacts with a half-moon icon next to their name signifies the user is idle, meaning they have either stepped away from the computer or have left Facebook open but not touched their account in a short while. These contacts may not receive your IM from Facebook Chat right away.