We ought to have the answer to this in the Philly mayor's race, where many of the candidates had some insane number of MySpace friends in relation to their actual support.
The totals were as follows, with their actual vote in parentheses:
Tom Knox 6,439 (70,043)
Chaka Fattah 4,568 (43,141)
Michael Nutter 888 (104,299)
Dwight Evans 250 (22,295)
Bob Brady -- (43,200)
Two of the candidates had about 10% of their supporters on MySpace (!). That's the equivalent of a Presidential candidate (in the general) with 6 million friends. The Philly mayoral candidates have to be judged as more successful than any of the Presidential candidates on MySpace.
Now, let's turn it around. Now that we've seen the upper limits of what MySpace can do, what impact did it have? I would have to assume that this would be a gold mine of new voters, particularly in primaries that tend to skew old.
MySpace didn't have the chance to swing the outcome because the margin of victory was so much greater than any of the candidate's friend counts. However, the winning candidate had less than 1% of his supporters on MySpace. Not only that, but he surged to victory on a wave of grassroots support at the end -- and he did it without MySpace apparently.
I do think that social networking can be *more* relevant in local races because of the local connection -- you're more likely to actually *know* the other people in the community. And it's easier to make a difference (one would think). Going by the numbers, candidates like Knox and Fattah should have used MySpace to great effect. Did they, and it just wasn't enough? Or did their MySpace friends fail to show up on Election Day?

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It's the community manager, not the tool that matters.
The long and short of it is the tools are irrelevant. Building a community and an online reputation is what is needed, and that is a different skill than making a video or having a MySpace page.
Candidates who play with MySpace and YouTube today aren't going to have time to learn Twitter tomorrow, and what happens when YouTube is replaced by VideoEgg or Eyespot or some other Web 2.0 software?
The key is simplicity - the whole point of Web 2.0 is forgetting the back end and focusing on content.
Hire a reliable social media consultant who has created online communities in the product marketing, PR, college, or music space, and let them decide what technology your local community will feel comfortable with. It may be a fancy Ajax interface in Seattle, but a simple blog in Kansas.
The important aspect is not what your platform is, whether MySpace, Facebook, blogs or a website, but how accessible that platform is and whether or not you're actively reaching out to the potential campaign evangelists in your district.
How do they want to be connected to you? How do they want to be organized? And how do you promote what you're doing, so it gets replicated by larger online groups who thank you with link and traffic attention?