This paper investigates the impact of Twitter attention, measured by abnormal number of tweets on stock trading activities. We find that Twitter attention has predictive power for future stock volatility and trading volume. A heightened number of tweets is followed by high volatility and trading volume over the next trading day. This finding is robust when focusing on international markets and controlling for other attention measures. We also find that high Twitter attention strengthens stock price adjustment to recommendation changes whereas it alleviates post-announcement price drift. These findings suggest that market underreaction to new information is related to limited investor attention from social media.