"VM" is defined as a "recent marketing phenomenon that motivates internet users to pass marketing messages voluntarily to their relatives, colleagues and friends". The main objectives of the study can be summarized in identifying the effect of viral marketing messages' exposure, attractiveness, incentives, credibility and confidence on consumers' purchasing decisions of fast food products. Another objective is to identify the contribution of demographic factors to the activation the messages' role in purchasing decisions. Based on an analytical descriptive approach, the field research was applied to 391 participants of Internet users. The collected data were analyzed by statistical techniques of descriptive statistics and Linear Simple Regression Analysis. The results showed a statistically significant effect of exposure rate, attractiveness, incentives and degree of credibility as independent variables on purchasing decisions as a dependent variable. Other 2 variables (level of confidence among parties of viral marketing messages and demographic differences among recipients) showed insignificant effect on the purchasing decisions.