Facebook is a social network very popular among University students for purposes of self-presentation. As social networks support the sharing of ideas, can we facilitate collaborative learning in Facebook? We designed a Facebook app that supports scripting of learners' interaction and the construction of arguments in Facebook. In an empirical study with 128 undergraduate teacher trainees, we investigated how individual preparation and argument structuring influences collaborative learning outcomes. The results show no significant effect of argument structuring and detrimental effects of individual preparation. Learners who were asked to individually construct arguments before joining a discussion in the Facebook app learned significantly less and diverged significantly more from their learning partner in learning outcomes.