This paper explores how the incorporation of a social network such as Facebook can enhance the acquisition of specialised vocabulary in the context of a Massive Open Online Course (MOOC). Such initiative took place in the second edition of the MOOC Professional English, the first ever English for Specific Purposes (ESP) MOOC to be launched in Spain as one of the courses offered by Aprendo, the UNED online platform. The main aim of the experiment was to ascertain how this social network, which has proved to foster motivation and engagement in language learning contexts (Blattner & Lomicka, 2012; Zourou, 2012), could enhance the students' learning experience and promote vocabulary acquisition in an ESP MOOC context. Following an action-research methodology (Lewin, 1946) a Facebook group was created by the MOOC curator and ran for eight weeks out of the twelve that the course was comprised of (11 November 2013-31 January 2014). A mixed-method approach was adopted for the data collection, using both quantitative techniques, such as student tracking in the MOOC, and also qualitative ones (e.g. questionnaires). The results point towards a positive impact of the Facebook network in the motivation of students to learn specialised vocabulary and an improvement in their progress in the MOOC, likewise fighting the main two problems that MOOCs currently are said to have: high drop-out rates and lack of student engagement. [For the complete volume, "New Perspectives on Teaching and Working with Languages in the Digital Era," see ED565799.]