One option is to ask a lawyer to draft a response in your name in which you clearly state that you have no connection to the offending service. You would then send that response on your personal letterhead. If FB's lawyers continue to pursue their claims, your lawyer would likely take the reins and communicate directly with the opposition on your behalf.

Another option is to remain silent and wait for a follow up from FB's attorneys before pursuing the letter strategy.

Bottom line is: consult a trained professional (lawyer). As someone who has been down a similar road, I can tell you firsthand that it may be painful but it's necessary. Remember, FB's legal budget is far larger than yours. You want to play the poor defenseless Rick card as long as you can before introducing your attorney and any legal letterhead.

Good luck!

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.