There are obviously different approaches depending on the size of your fan base. If you have a large audience, you might not be able to put a lot of time into each lead. Your overall goal will likely be about brand awareness and creating buzz for your company. You should consider metrics that capture overall influence such as retweets, blog posts and clicks.
If you have a targeted audience, each lead probably carries more value. You overall goal will likely be to generate new leads. You should consider metrics like sales, email sign-ups, @messages, direct messages and clicks.