  Despite its undisputed position as the biggest social media platform,
Facebook has never entered the main stage of altmetrics research. In this
study, we argue that the lack of attention by altmetrics researchers is due, in
part, to the challenges in collecting Facebook data regarding activity that
takes place outside of public pages and groups. We present a new method of
collecting aggregate counts of shares, reactions, and comments across the
platform-including users' personal timelines-and use it to gather data for all
articles published between 2015 to 2017 in the journal PLOS ONE. We compare the
gathered data with altmetrics collected and aggregated by Altmetric. The
results show that 58.7% of papers shared on Facebook happen outside of public
spaces and that, when collecting all shares, the volume of activity
approximates patterns of engagement previously only observed for Twitter. Both
results suggest that the role and impact of Facebook as a medium for science
and scholarly communication has been underestimated. Furthermore, they
emphasise the importance of openness and transparency around the collection and
aggregation of altmetrics.
