Facebook use different type of communications and different way,

Chat (ID@chat.facebook.com) <-> Chat (ID@chat.facebook.com)
Email (loginname@host.com) <-> Message (username@facebook.com)
Email (email@host.com) <-> Message (username@facebook.com)
Email (email@host.com) -> Wall - Mobile Upload (name000name@m.facebook.com)
Email (loginname@host.com) -> Group Post (ID@group.facebook.com)
ID (user ID) <-> ID (user ID)
ID (user ID) <-> fID (ID@facebook.com)
ID (user ID) <-> Group Post (group ID)
and so on...