LAST Wednesday, more than 100,000 people showed up in Istanbul for a funeral that turned into a mass demonstration. No formal organization made the call. The news had come from Twitter: Berkin Elvan, 15, had died. He had been hit in the head by a tear-gas canister on his way to buy bread during the Gezi protests last June. During the 269 days he spent in a coma, Berkin’s face had become a symbol of civic resistance shared on social media from Facebook to Instagram, and the response, when his family tweeted “we lost our son” and then a funeral date, was spontaneous.

Protests like this one, fueled by social media and erupting into spectacular mass events, look like powerful statements of opposition against a regime. And whether these take place in Turkey, Egypt or Ukraine, pundits often speculate that the days of a ruling party or government, or at least its unpopular policies, must be numbered. Yet often these huge mobilizations of citizens inexplicably wither away without the impact on policy you might expect from their scale.

This muted effect is not because social media isn’t good at what it does, but, in a way, because it’s very good at what it does. Digital tools make it much easier to build up movements quickly, and they greatly lower coordination costs. This seems like a good thing at first, but it often results in an unanticipated weakness: Before the Internet, the tedious work of organizing that was required to circumvent censorship or to organize a protest also helped build infrastructure for decision making and strategies for sustaining momentum. Now movements can rush past that step, often to their own detriment.

In Spain, protesters who called themselves the Indignados (the outraged) took to public squares in large numbers in 2011, yet the austerity policies they opposed are still in effect. Occupy Wall Street filled Lower Manhattan in October 2011, crystallizing the image of the 99 percent versus the 1 percent without forcing a change in the nation’s widening inequality. And in Egypt, Tahrir Square protesters in January 2011 used social media to capture the world’s attention. Later that year, during clashes in the square, four people in their 20s used Google spreadsheets, mobile communication and Twitter to coordinate supplies for 10 field hospitals that cared for the wounded. But three years later, a repressive military regime is back in power.