In this paper we explore how individual personality characteristics influence online social networking behavior. We use data from an online survey with 1560 respondents from a major Swiss technical university and their corresponding online profiles and friendship networks on a popular Social Network Site (SNS). Apart from sociodemographic variables and questions about SNS usage, we collected survey data on personality traits with a short question inventory of the Five Factor Personality Model (BFI-15). We show how these psychological network antecedents influence participation, adoption time, nodal degree and ego-network growth over a period of 4 months on the networking platform. Statistical analysis with overdispersed degree distribution models identifies extraversion as a major driving force in the tie formation process. We find a counter-intuitive positive effect for neuroticism, a negative influence for conscientiousness and no effects for openness and agreeableness.