Online petitions often have no outlined plan of action. No one knows where they ultimately go, which may be nowhere, or who the petition will be given to and when. Politicians, who are in a position to do something about an issue, have no reason to pay attention to Internet petitions. They don't know if the people involved are their own constituents or even if they actually exist.

On the Other: They Can Indicate Public Opinion

If many people rally behind a cause, it can garner publicity and therefore affect change. Popular online petitions can also be seen as a barometer of public opinion. For example, an online petition was begun in 2008 to stop critically lambasted filmmaker Uwe Boll from making any more movies. An article in the British newspaper "The Guardian" provided so much publicity about it that Boll promised to retire if the petition received a million signatures.

Bottom Line

Simply making or signing an online petition does not guarantee that the petition will work. Politicians and other people in positions of influence have no idea whether or not the signatures on the petitions are real, and often nothing is done with the petitions at all. However, they can be good barometers of public opinion and therefore can attract attention to the issue involved if enough people participate and gain it some publicity.

Source:

Barbara Mikkelson: Internet Petitions

Guardian article: Director Promises to Retire if a Million People Demand It