Building a new social network (SNS) with connections is especially important for first-year students to deal with the transition to college. The goal of the current study was to investigate the link between congruence in extroversion and the interaction frequency within cross-sex classmate dyads on SNS. Based on a cross-sectional study among 371 cross-sex first-year students dyads (M-age = 18; SD = 2.08) using polynomial regression with response surface analysis, we found that: (i) dyads with congruent levels of extroversion indeed interacted more frequently online than dyads with incongruent levels of extroversion; (ii) extroverted dyads interacted more frequently online than introverted dyads; and (iii) dyads with an increased level of incongruence on extroversion had a decreased frequency of online interaction. The sex-segregated social network might be magnified by the difference in extroversion within cross-sex dyads. The discussion focused on potential explanations from the reward of interaction model and the expectations for cross-sex friendship.