The message feature allows a user to send a message to a friend. This is not intended to be a full-fledged chat feature but rather a simple way of sending a notification to a friend. For example, a developer could create and invite someone to a game by allowing a user to send an invite to a lobby, with the necessary information to join that lobby.

A message is only sent to users that have their availability as ONLINE, BUSY, or AWAY. The message must also be a serializable object.