you are on a gray area. Meaning that if this is against their TOS they could still threaten or sue you. Never underestimate big bullies. Now if they would actually win is hard to say. Sites dedicated on exploiting something in another big site have been sued in the past. Their argument in court would be that you profit from their name/business. PS: I assume at least you won't name the site facebookproxy.com. That would be trademark infringement