Some companies are creating a presence on Facebook as part of a corporate strategy. These companies might value employee input and contributions to the Facebook page. If that's the case with your workplace, you might be expected to befriend coworkers. That might include your boss.
Rather than be intimidated by this, consider it an opportunity to show your own value to your employer. You'll probably need to maintain a professional appearance on the social network. Use Facebook's privacy settings and filter options to limit what people can see on your profile. You can even sort your coworkers into a list that only has access to certain parts of your page. Keep those photos of you wearing the lampshade and dancing the hula for people who don't sign your paycheck.