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.

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