People today turn to social media for expressing their views and opinions. Often these opinions trigger large-scale discussion and collective mobilization. However, the impact of such massive discussion and mobilization over social media is unclear. In this study we aim to examine and understand this impact by seeking answer to the following question: Under what conditions do social media mobilization influences Government to respond? We examine this phenomenon using the theoretical lens of critical mass theory and media affordances. The results of this study suggest that government responds to citizen’s online activism when a critical mass is generated. By analyzing the tweets of 100 citizen-led online activism events, we found that supporters and amplifiers play a crucial role in pressurizing the government to respond.