1. If you've already added a user as a friend, go to your profile page to see a listing of your current friends. (If you've not yet added your friend, see this post for instructions on how to do so: How do I search for and add friends?)
To see your list of friends, go to the Planet menu and choose My Planet > Friends:
2. When you see your Friends listing, click on the Start Conversation link to the right of any friend's profile picture to start a conversation.
(Click on this image to enlarge)
3.You'll then see the Start a Conversation box. Enter your message and click on Start Conversation to send your friend a message. Your friend will then receive an e-mail in his or her e-mail box with the option to reply to the conversation.