To follow someone on Facebook without friending them, use the "Follow" button on that person’s Timeline or profile page; just click on this and you’ll be following that person without friending them. However, you’ll only find one of these if the person’s privacy settings allow this and it won’t be there if the person has set these to disallow non-friend follows (this setting is under "Who Can Follow Me" and needs to be set to "Everyone" for follows to work without friending).