They sent you the letter because for some reason, in some way, they tracked the post to you and determined that it was your post--possibly you included some details in it which identified you.As to why they sent you the letter:1) A person who posts something online, even if they ostensibly do so "anonymously," is responsible for what they post; if they defamed someone, for example, they can be sued over it, or if they disclosed protected confidential information, they could be sued for that disclosure.2) Federal law protects the website from liability: a website is not liable for what other people post on it, so long as the website did not help create the post. If the website is simply being a "conduit" for your posts, you, not they, are liable under the law. Therefore, they sent the letter to you because you are the one who is legally responsible for this post. If you do not comply, if they have a valid cause of action (e.g. if you defamed someone), you could be sued.