Facebook directly don't give you any option to send a message to all your friends, It gives restriction of 20 people, so it would consume a lot of time in sending messages to all your friends, and even if you want to send group of same people again a different message, just send them again 20 people at once, a waste of time!.
So I found out a innovative method for you to send message to all your Facebook friends, and that too by using Facebook only, it is easy to send messages through the Events Function (WITHOUT the restriction of 20 people).
Update- Ability to send messages to the event guest is no longer available,