The research investigates the factors leading to user loyalty of Web 2.0 sites. Survey of users in a popular Web 2.0 site in Taiwan was conducted. Results show that sense of belonging and service quality have direct effect while trust affects loyalty indirectly via sense of belonging. In addition, service quality also affects loyalty through trust and sense of belonging. One surprising finding is that trust has non-significant direct effect on loyalty as originally proposed. Another interesting finding comes from light user group. Among light users, service quality has no direct effect on loyalty, making sense of belonging the only direct impact on loyalty.