That's a hard question to answer without providing some information on your product/service. Il raises a good question in his post, 'why do you want Facebook fans?.' That answer will help determine how you want to gather those FB fans, because there are a lot of ways to do so and some are not so legit.

But, my short answer would be to leverage your existing product/service to convert your active community into FB fans. Those will most likely be your most engaging FB fan audience and easiest to become fans with the right incentive. From there, your networks of fans will only start to grow.

Sorry I couldn't give you a specific answer, but without knowing your business I couldn't provide a suggestion. In my own community, we encourage the players of our games to become facebook fans through virtual currency, bragging rights, and other incentives.