Facebook is a great way to find old friends and snoop to see what they're up to. It's also a great way to find information about clients, potential clients, and witnesses. Facebook's Graph Search allows you to search for all posts about topics (e.g., Ohio State football), things people "like" (e.g., recreational drug use), places they've been (e.g., Franklin County Court of Common Pleas), and more. Once you have that first group identified, you can sort or filter the results by people's marital status, employer, hometown, etc. It's a little clunky, but it takes just a few minutes of tinkering.
Unrelated to the Graph Search, you can also find photos of people on Group or Event pages, regardless of whether the person has high privacy settings. Let's say, for example, you know an event like a 10K race happened on a particular date. Simply go to the Group or Event page and scroll through the photos, and you just might find photographic evidence that a particular person was in attendance.
One last Facebook research tip: though Facebook has "helpfully" allowed you to edit your comments, it retains the edit history and anyone can click on "show edit history" to see the changes you've made. Consider deleting any comments you've written that are incorrect or embarassing.