In the past, activists had to spend months or years building the organizational structures and links necessary to launch a mass protest in the real world.
This also made movements long-lasting, since they were supported by the infused discipline and order. But, nowadays social media allows potential protesters to bypass those steps, driving one another to action with as little as a viral post.
This usually leads to rallies that bring thousands or millions of bodies to the street in a flash, but that often fade away just as quickly.