There is an article in PNAS which may help understand the secret of facebook et al: "Disclosing information about the self is intrinsically rewarding" by Diana I. Tamir and Jason P. Mitchell in PNAS. The abstract reads:
"Humans devote 30–40% of speech output solely to informing others of their own subjective experiences. What drives this propensity for disclosure? Here, we test recent theories that individuals place high subjective value on opportunities to communicate their thoughts and feelings to others and that doing so engages neural and cognitive mechanisms associated with reward. Five studies provided support for this hypothesis. Self-disclosure was strongly associated with increased activation in brain regions that form the mesolimbic dopamine system, including the nucleus accumbens and ventral tegmental area. Moreover, individuals were willing to forgo money to disclose about the self. Two additional studies demonstrated that these effects stemmed from the independent value that individuals placed on self-referential thought and on simply sharing information with others. Together, these ﬁndings suggest that the human tendency to convey information about personal experience may arise from the intrinsic value associated with self-disclosure."