The purpose of this study was to find out the effect of trust, security, quality of service and perceived risks on online purchasing decisions. To investigate thehypotheses of the research, data were collected through distributing questionarre with a samplesize of 385 samples.The questionnaire consisted of a list of statements submitted to respondents to obtain data relating to research. The methodology was conducted usingStructural Equation Modeling (SEM) program, and LISRELsoftware to perform factor analysis. The study revealed that trust variable on online Purchasing Decisions (PD) had positive and not significant effect with t-value of 1.38 and the estimated value of 0,19. The effect of Security Variable on Purchasing Decisions (PD) had a positive and not significant effect with t-value of 0.87 and estimated value of 0.12. The effect of Quality of Serivice (QS) on Purchase Decisions (PD) had a positive and significant effect with t-value of 6.03 and estimated value of 0.37. The effect of the Perceived Risk Variable on Purchasing Decisions (PD) had a positive and significant effect with t-value of 5.82 and estimated value of 0.44. The Quality of Serivice (QS) variable had a dominant effect on online purchasing decisions through Social Networking Websites compared to the Perceived Risk variables, Trust Variables and Security Variables