This study analyzes factors that influence customers' decision making process in the context of social networking websites. With the use of "Facebook Store App" as an example, experimental method is adopted to allow manipulation of four groups divided by tie strength (strong/weak) and perceived risk (high/low). The results show that significant difference exists between tie strength and perceived diagnosticity; the level of perceived risk has interference effect on tie strength and perceived diagnosticity; perceived diagnosticity positively influences customers' behavior intention; flow has positive moderating effect on perceived diagnosticity and purchase intention. The study suggests that in the context of social networking sites, businesses should value the importance of strong ties on customers' decisions. Low perceived risk products are more suitable to be sold.