We're actually looking at implementing Susan's approach. Facebook is one of our top website traffic referrers, even with our limited number of fans. We get good engagement to it (that and Ben tends to share stories with his personal page as well).
As for more specific ads, it's hard to top the engagement of the motorcyclist community (maybe cyclist would one specific target?). Being honest, I don't think that most people not looking for a lawyer would engage with a lawyer ad unless it's really provocative. It would have to have a "scare" headline like: You May Be Risking Financial Ruin - and have that be about not getting enough underinsured and uninsured motorist coverage. Have it target married people over the age of 40. Just a thought.
Also, we don't actually have any niche Facebook pages - we would have one if we did motorcycle accidents, but I'm just not sure about them outside of those really engaged communities. What kind of engagement do you all get from your niche pages?