To organize an effective protest, you need structure and people who are truly committed to the cause -- willing to be arrested or killed. Social networking provides the exact opposite.
History has shown that you're best off working with people you're close to -- they're the ones who will listen to your crazy ideas. Invite 500 of your Facebook friends to a protest, and 498 will RSVP "maybe" (meaning no) and the two who show up will only be there because they want to see you get your ass kicked by the riot police.