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 causal role in driving up these prices, however, remains unclear. To address these questions, we employ causal inference techniques, leveraging data capturing activity on Reddit and Twitter, and trading volume with a high temporal resolution. We find that Reddit discussions foreshadowed trading volume before the GameStop short squeeze, with their predictive power being particularly strong on hourly time scales. This effect emerged abruptly and became prominent a few weeks before the event, but waned once the community of investors gained widespread visibility through Twitter. As the causal link unfolded, the collective investment of the Reddit community, quantified through each user's financial position on GameStop, closely mirrored the market capitalization of the stock. The evidence from our study suggests that Reddit users fueled the GameStop short squeeze, and thereby Reddit served as a coordination hub for a shared financial strategy. 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.

翻译：2021年初，GameStop、AMC、诺基亚和黑莓的股价出现急剧上涨，这一现象由轧空操作引发，而该操作被广泛归因于Reddit上的散户投资者。这些事件首次展示了在线社交网络催化金融集体行动的潜力。然而，Reddit用户究竟如何、何时以及在多大程度上对这些股价上涨起到了因果作用，仍不清楚。为解答这些问题，我们采用因果推断技术，利用高时间分辨率的数据捕捉Reddit和Twitter上的活动以及交易量。我们发现，在GameStop轧空事件发生前，Reddit上的讨论已预示了交易量，其预测能力在小时时间尺度上尤为显著。这种效应突然出现，在事件发生前几周变得显著，但一旦投资者社区通过Twitter获得广泛关注后便逐渐减弱。随着因果关系的展开，Reddit社区的集体投资（通过每位用户在GameStop上的财务头寸量化）与股票市值紧密对应。我们研究的证据表明，Reddit用户助推了GameStop轧空事件，因此Reddit充当了共同金融策略的协调枢纽。至1月底，讨论GameStop的用户提升了黑莓、AMC和诺基亚的热度，这些股票成为社区获得全球认可后最受欢迎的股票。总体而言，我们的结果揭示了社交媒体用户驱动的首次大规模金融集体行动背后的动态机制。