  In early 2021, the stock prices of GameStop, AMC, Nokia, and BlackBerry
experienced dramatic increases, triggered by short squeeze operations that have
been largely attributed to Reddit's retail investors. These events showcased,
for the first time, the potential of online social networks to catalyze
financial collective action. How, when and to what extent Reddit users played a
role in driving up these prices, however, remains unclear. We address these
questions by statistical analysis of time series with high temporal resolution,
about social activity on Reddit and Twitter as well as stock trading volumes.
We find that increasing Reddit discussions anticipated high trading volume
before the GameStop short squeeze, with their predictive power being
particularly strong on hourly time scales. This effect emerged abruptly a few
weeks before the event, but waned once the community of investors gained
widespread visibility through Twitter. Meanwhile, the collective investment of
the Reddit community, quantified through each user's financial position on
GameStop, closely mirrored the market capitalization of the stock. These
evidences suggest a coordinated action by Reddit users in developing a shared
financial strategy through social media. Towards the end of January, users
talking about GameStop contributed to raise the popularity of BlackBerry, AMC
and Nokia, which emerged as the most popular stocks as the community gained
global recognition. Overall, our results shed light on the dynamics behind the
first large-scale financial collective action driven by social media users.
