I used to think that online petitions were useless. But now I think they serve a purpose in that they provide a way for people to raise the topic with others and get people thinking about it. It's awkward to say to people out of the blue "Hey, I'm concerned about this issue. Have you given it any thought?" But it's socially acceptable to say "Hey, I've signed this petition. Do you want to sign it?" Which is exactly what I've done in this thread.
Of course, a petition alone probably isn't going to change a politician's mind about how to vote. But it might get them worried enough that they will canvas their constituents about it.
Also, people who sign online petitions may be doing other things as well, like donating to the
EFF, but they don't necessarily talk about it.

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