As a non-human entity, facebook is neither good nor evil. However, many people disregard the implications of posting the most intimate and secret details of their lives for the world to see.
Employers now screen facebook profiles to determine the acceptability of applicants and children especially get into trouble by accessing and posting to facebook. There was a law introduced to streamline the process of changing a child's legal name when the child grows up because of all the dirt left behind on facebook. Child use of facebook continues to grow even though facebook places stringent age restrictions on account creation.
Therefore, facebook is neither good nor evil but some uses of facebook are extremely sinaster.