In the past few weeks and months, social media have helped many groups unleash widespread advocacy efforts that have won the attention of millions of people and ignited large-scale public debates.

Supporters of Planned Parenthood, for example, used Facebook and Twitter to inspire protests after Susan G. Komen for the Cure said it would end support to the organization. Invisible Children used a YouTube video to start a campaign to make the public aware of abuses committed by the warlord Joseph Kony, in Africa.

And the killing of Trayvon Martin in Florida has fueled a massive online debate about race relations and gun laws and sparked large protests nationwide.

What is it that makes these events spread online and generate support?

The common thread is that the organizers found a way to help anyone relate to the cause, says Stephanie Rudat, an online activist and co-founder …

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