Facebook and MySpace were rated as having good security.  However highly locked down settings are required to achieve this.  Users must alter settings beyond original defaults and exercise best practices while connected.  

Zone Alarm evaluates security for 8 Social Networking sites

QUOTE: Deciding which social networking sites to avoid or use can be a challenge. Discovering what privacy settings they have, or don’t have, can be even more cumbersome. Here, we rank eight of the most popular social networking sites according to their privacy and look at some of the most famous security attacks from 2010.