From the facebook FAQ:

"The left column of your profile may display two types of friend lists: Friends By default, a changing selection of friends always appears under the "Friends" heading in the left column of your profile. The selection includes many friends who you publicly interact with the most. Examples of public interactions include Wall posts, comments and mutually attended events Facebook will never show friends based on whose profiles you choose to view or who you interact with over messages and chat. Feature specific friends or groups In addition to your general list of friends, you can choose to feature specific friends (e.g., best friends, roommates or coworkers) or groups to which you belong. Learn how to do this here. /help/?faq=19427"

You're welcome