Social Network Services (SNSs) have grown to be new and promising tools of marketing. By referring to researches done on e-mail viral marketing, this paper operationizes SNS viral marketing effectiveness to accurately reflect marketing success in SNS environment, and tries to identify its affecting factors. As potential affecting factors, fan size, advertisement type, existence of engagement elicitation and incentive are identified. By sampling real advertisement postings on Facebook, we showed that fan size, advertisement type, and engagement elicitation are factors affecting SNS viral marketing success. This research expanded the conventional model of viral marketing into SNS settings to improve understanding on SNS viral marketing. Motivation is discussed as an important factor, and this research showed that viral campaign can be more successful when it triggers internal motivation to engage, but not the external motivation. This research could also be a guide for practitioners on how to post a successful advertisement in SNSs.