This study focuses on the effect of cyberostracism on social networking sites. Based on the temporal need-threat model of ostracism, we examined a) reflexive reactions, specifically worsened mood and threats to the four fundamental needs (i.e., belongingness, self-esteem, meaningful existence, and control), and b) reflective reactions, in the form of prosocial, antisocial, and avoidance behavior. We also focused on the role of social anxiety. Using the experimental tool Ostracism Online, we conducted an online experiment to manipulate ostracism, measure self-reported reflexive reactions, and measure reflective reactions in a newly developed cooperative financial game. The participants were 196 young Czech adults (age 18-30; 62% women). T-tests showed worsened mood and higher threat connected to all four of the fundamental needs in the reflexive stage in ostracized participants. Regression models showed that social anxiety had a small effect on reflexive reactions, but it did not moderate the effect of ostracism. The type of threatened need and social anxiety did not predict a reflective reaction. The only significant predictor of antisocial response was experienced ostracism. Even a mild form of ostracism such as the lack of reactions by strangers to a shared post can lead to negative emotional and behavioral consequences.