This study attempted to compare the effectiveness of employing Facebook and traditional instruction in improving Iraqi EFL secondary school students' vocabulary knowledge. The factors that positively and negatively affect the EFL students' vocabulary learning were also investigated. The study employed a quasiexperimental design with two groups; the control and the experimental groups. In the control group, thirty-six students were taught English vocabularies via the in-class conventional method, and the experimental group which consisted of another thirty-six students were taught using Facebook as an Online Learning Platform (FOLP) to enhance their knowledge of English vocabularies. A mix of quantitative and qualitative research methodology was used where triangulation of pretest, posttest, semi-structured interview and observation were used. Results showed that FOLP was more effective than traditional instruction in improving EFL students' vocabulary knowledge. In addition, results indicated that a variety of teaching techniques, effective group work, immediate feedback, and autonomous learning opportunities are factors that positively affect students' learning of English vocabularies. Moreover, students perceived that domination of the good students of the discussion, technical problems, and social loafing are the factors that negatively affect their learning of English vocabularies. The results confirmed the liability of using Facebook in broader contexts.