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It all started in the 1990s and has continued to grow, most recently as a driving force in the London riots. With the Web 2.0 movement and social networking boom of recent years, just about everyone has a voice via the Internet -- and many are using theirs to mobilize.
Here is a timeline of some key moments in online activism, most of which resulted in a physical protest or directly affected change. Do you remember where you were at these moments in history? Share your story in the comments below.
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Mashable community team members Stephanie Haberman, Todd Olmstead, Connie Preti and Chelsea Stark contributed to this report.