Who do you want your ad to be viewed by? Facebook offers unparalleled options for targeting, based off of information that users put on their own profiles, as well as their actions on Facebook and other sites.
If you already know your audience, you can target them effectively. Facebook allows you to target based on location, age, gender, interests, and much more. While it seems like a no brainer to target the largest groups possible to reach the biggest audience, this is actually a common mistake.
In fact, rather than targeting broad terms like "cameras" or "baseball", it's best to target niche and specific interests. For example, choosing a type of camera, like Nikon or Olympus, or a specific baseball team like the Yankees or Red Sox, gives you a much better chance of reaching people who are actually going to take action on your ad.
If you don't already know your audience, you should learn about them rather than guessing. One way to do that is to create a customer survey to gain the necessary information to place a targeted ad.