I your place I would avoid going the legal route since this is not your strength.
1. Pay close attention to comments from daegloe
> EDIT: After reading many of the frothy comments on this page, I just wanted to add that it's generally best to avoid threatening a lawsuit, countersuit or any kind of legal claim in your response. These giant law firms love to call people on their bluffs, because they bill by the hour. Avoid boxing yourself in.
> Best strategy is to avoid litigation at all costs. Because, well, it costs lots of money whether you win or lose. And if you win, collecting is a bitch in of itself.
2. Do you have deadline in the cease and desist letter? If yes, what's the deadline?
3. Since this is a typical "David vs Goliath" and media loves these types of stories, I would try defeat FB with its own baby - social media.
Specifically, I would
3.1. contact 10-15 major tech blogs, including through Twitter
3.2. contact 5 major newspapers/their journalists
3.3. post your story on FB Twitter, FB Wall, Mark Zuckenbergs Wall
4. I am sure that this had adverse impact on your health and well being. So I would probably add sharing those to your story.
Play to your strengths and keep us posted.