Oh... ugh, sorry this is happening to you. I've had this happen to me a few times, both with wikis and personal fansites lifting stuff from my site(s) word for word. I've actually had almost my entire site lifted before too.

Being that it's on the PoT Wikia where you don't even know who did it, as opposed to someone else's "personal" fansite, I'd ask whether you mind having what you wrote over there. Once upon a time, I'd most likely jump in and go on a deleting rampage just for the mere principle alone. If you decide to leave it there, I'd definitely go in and add a source note though.