How do you determine if a social media campaign is successful?

[Michael]

This depends on how you are handling your social media.

Are you using it primarily for branding, or are you linking offsite to blogs, websites, etc?

If you are linking to a website, Google analytics can tell you where traffic is coming from, which for the social networks can tell you how effective specific linkings have been.

It's more difficult to get quantitative data if you're a business trying to get people physically through the door; one trick a friend used to use is to create "event pages" for social networks, do research on what confirmed guests look like, and see if they actually show up. Obviously a very mom and pops strategy, but it worked even better when he knew who they were walking through the door!