Jean Twenge is a psychology professor at San Diego State University, a co-author of "The Narcissism Epidemic" and the author of "Generation Me."

If you have a thought you’d like to share, it’s no longer necessary to wait: Post it to Facebook or Twitter, and an audience is yours in an instant.

Narcissism clearly leads to more social media use, social media use leads to positive self-views, and people who need a self-esteem boost turn to social media.

Is this ability making us narcissistic – overly focused on ourselves, with an outsized vision of our own influence? As Keith Campbell and I document in "The Narcissism Epidemic," narcissism is rampant in our current culture, from plastic surgery to reality TV to young people’s rising narcissism scores. But is social media the cause?

It might be. Many studies find that narcissists have more friends on Facebook and post more, especially provocative material. They also post more often on Twitter. Apparently, narcissists thrive on social media. As one paper concluded, narcissists use Twitter “as a kind of technologically augmented megaphone: A means of amplifying one’s own perceived superiority to others.” They use Facebook as “a technologically enhanced mirror, reflecting a preoccupation with one’s own image, others’ reactions to this image, and a desire to update the image as frequently as possible.”

But does social media use actually cause narcissism? For that, you need an experiment – a random assignment to spend time on social media vs. another site. Two experiments show that spending time on Facebook caused higher self-esteem, while using MySpace (but not Facebook) caused higher narcissism. In another experiment, those who received a blow to their egos were more likely to want to use Facebook.

In sum: Narcissism clearly leads to more social media use, social media use leads to positive self-views, and people who need a self-esteem boost turn to social media. It is less clear whether social media directly causes narcissism, at least in the short term. With narcissists having more friends and posting more frequently, however, social media sites are clearly influenced by those high in narcissism at a rate higher than their fair share. And that’s just the way they like it.