Abstract: In Facebook, we studied the relationship between sociometric popularity (number of Facebook contacts) and personality traits. We tested to which extent two prevalent viewpoints hold. That is, sociometrically popular Facebook users (those with many social contacts) are the ones whose personality traits either predict many offline (real world) friends or predict propensity to maintain superficial relationships. We found that the strongest predictor for number of friends in the real world (Extraversion) is also the strongest predictor for number of Facebook contacts. We then verified a widely held conjecture that has been put forward by literary intellectuals and scientists alike but has not been tested: people who have many social contacts on Facebook are the ones who are able to adapt themselves to new forms of communication, present themselves in likeable ways, and have propensity to maintain superficial relationships. We will see that there is no statistical evidence to support such a conjecture. In Twitter, instead, we tested whether users can be reduced to look-alike nodes (as most of the spreading models would assume) or, instead, whether they show individual differences that impact their popularity and influence. Again, one aspect that may differentiate users is their character and personality. The problem is that personality is difficult to observe and quantify on Twitter. It has been shown, however, that personality is linked to what is unobtrusively observable in tweets: the use of language. We thus carry out a study of tweets and show that popular and influential users linguistically structure their tweets in specific ways. This suggests that the popularity and influence of a Twitter account cannot be simply traced back to the graph properties of the network within which it is embedded, but also depends on the personality and emotions of the human being behind it. After introducing both studies, we will discuss theoretical implications of our findings as well as practical implications for viral marketing and social media security.