Spending too much time on Facebook may damage more than a teen's eyesight.

Young adults who are constantly logging onto the world's largest social networking Web site are more likely to display narcissistic tendencies as well as a slew of other psychological disorders, according to Larry D. Rosen, a professor of psychology at California State University, Dominguez Hills.

Rosen -- who spoke of Facebook's potentially damaging effects at the 119th Annual Convention of the American Psychological Association -- said new research has found that teens with a strong presence on Facebook may exhibit potentially adverse effects such as antisocial behavior, mania and depression. In addition, the variety of distractions offered by Facebook may hurt learning and lead to lower grades in school.

However, Rosen said the news for young Facebook users isn't all bad.

"While nobody can deny that Facebook has altered the landscape of social interaction, particularly among young people, we are just now starting to see solid psychological research demonstrating both the positives and the negatives," Rosen said in a statement.

For instance, while researchers found that middle, high school and college students who logged onto Facebook at least once during a 15-minute study period were more likely to receive lower grades on exams, another study determined that online social networking can benefit shy or anxious teens by allowing them to virtually socialize with their peers through the relative safety of a computer screen.

Plus, researchers found that heavy Facebook users are better at showing "virtual empathy" to their online friends, an ability that may indicate they're more able to empathize with others in real-world situations as well.

Although some parents may be tempted to stealthily monitor their children's' social networking use to steer them clear of bullying or other inappropriate behavior, Rosen advices against the move.  Instead, he said parents should seek to build a sense of trust with their kids by discussing appropriate internet use early on and encouraging them to communicate any problems they may encounter while surfing the Web.

"You need to talk to your kids, or rather, listen to them," Rosen said. "The ratio of parent listen to parent talk should be at least five-to-one. Talk one minute and listen for five."