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.
Good idea. Before you get him to draft the letter though, you should try to negotiate a flat fee for the service. You don't want some guy that bills $x00/hr spending a few days on the letter.
The letter should be an hour discussion plus an hour of drafting, at the most. @vellum is right, always negotiate fees and estimated level of effort up front. In these types of cases it's generally best to deal with each task discretely until, in a worst case, you must consider litigation. This helps avoid locking yourself into a significant retainer when a single letter might do the trick.
I am not a lawyer, but briefly consulting a lawyer might also allow you to find out if it's possible to litigate (countersue?), and thereby recoup your legal fees.
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.
