Everything has a cost somehow, and with facebook, it may be more than you think or understand. Check out these articles on the dangers (potential costs) of using facebook.
http://www.csoonline.com/article/676819/social-networking-security-4-reasons-why-facebook-and-vanity-don-t-mix
http://www.csoonline.com/article/603441/facebook-dislike-button-scam-spreads-virally
http://www.csoonline.com/article/679808/security-firm-to-facebook-clean-up-your-act