While the majority of people using these sites do not pose a threat, malicious people may be drawn to them because of the accessibility and amount of personal information that's available. Predators may form relationships online and then convince unsuspecting individuals to meet them in person. Also your personal information can also be used to conduct a social engineering attack.
Additionally, because of the popularity of these sites, attackers may use them to distribute malicious code. Sites that offer applications developed by third parties are particularly susceptible. Attackers may be able to create customized applications that appear to be innocent while infecting your computer or sharing your information without your knowledge.