Despite the positive claims on the pedagogical use of social annotation and online collaborative writing tools discussed in the literature, most of the findings are derived from interviews or self-reported survey data. Very few studies probed deep into the learning processes and examined students' digital traces and the artefacts they co-construct. In this study, we employed semantic network analysis techniques to examine how the use of a social annotation tool (Diigo) coupled with an online collaborative writing (Google Docs) affects students' learning outcomes. The results indicate that the use of Diigo coupled with Google Docs helps enhance student engagement in the collaborative process and that the concept connectivity and quality of the text co-constructed by each group using Diigo coupled with Google Docs is significantly higher than those using Moodle's forum. In addition, the level of collaboration within a group correlates positively with the number of vertices with high lexical relevancy identified in the semantic network of the text co-constructed by each group.