To send the message, first click on someone’s name in the “people who like this” box on the left side of your page. If you aren’t friends with the person, you’ll see a little blue box at the top of their profile with a lock icon. Most people will have a text link in that box that says “send him/her a message”, which you can click even if you aren’t friends on facebook. Most people don’t realize that most of the time (depending on the person’s privacy settings) you don’t need to be friends with someone to message them. So thank away!