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 There are three main forms of hijacking in protest.  First, a group may try to hijack your protest to use it for their own purposes, or to discredit you and your group.  Second, an organization or group may try to hijack a person and use them for their purposes.  Third, a website, facebook, or comments section may be hijacked by those trying to subtly undermine the purpose of your group or to stem the flow of useful communication or to discredit those contributing productively.