A very recent study has shown that spending too much time on Facebook can cause Narcissism.
What's Narcissism Exactly?
According to Wikipedia, Narcissism or Narcissistic Personality Disorder "often means inflated self-importance, egotism, vanity, conceit, or simple selfishness. Applied to a social group, it is sometimes used to denote elitism or an indifference to the plight of others. [...] Today, in psychology, narcissistic personality disorder is a mental illness characterized by a lack of empathy, a willingness to exploit others, and an inflated sense of self-importance. In popular discourse, "narcissism" is a widely-used term for a range of selfish behaviors."
What does using Facebook have to do with acquiring this personality disorder?
Larry Rosen, a psychologist at Cal State Dominguez Hills who conducted this particular study on Facebook use and who's been studying the effect of technology on people for more than 25 years now, found out that --
[...] [S]ocial network feeds into a narcissist’s M.O. perfectly by allowing people to broadcast themselves 24/7 on their own terms, Rosen says. Among users of all ages, the more people used Facebook, the more likely they were to have antisocial personality disorder, paranoia, anxiety and alcohol use.
Source: Los Angeles Times