The success of online communities depends on different aspects and has been subject of several evaluation works. Most of the existing works in this scope are not generalizable due to their strong dependence to the considered community characteristics. Face to this finding, our aim is to propose a generic approach to evaluate and to improve the success of online communities. This paper starts by an identification of the success determinants of online communities including the participants, the technology as well as the common goal. After that, we propose a generic evaluation approach based on two levels. The first level consists in a failure detection based on a quantification of the success determinants. The second level focuses on a failure explanation and an identification of the most plausible causes of the detected failures. This approach was applied to evaluate the success of a social network-based community; it gave reliable results.