Once you are a follower, you can leave fan mail. There will be a notice that says, "You are now a follower of xxxx" - you will see written in blue - "post fan mail to xxxx".
Note: If you leave the page that says "post fan mail", and want to go back later:
1) Click on the Hubber's Profile page
2) Scroll down and you will see an empty box that says, "Post Fan Mail".....
3) Type in your message, and click any accolades you want to give......good writer, thank you, etc......
4) Click Submit.
Hope this helps!