Hey kongqiu, this was actually a bug on our side. Until today, we were in closed beta where we required users to have at least one friend on the service.
We removed that requirement last night. Unfortunately we left in a piece of code that redirects you to the thank you page if the login fails. Facebook Auth has been failing intermittently for us all day, so that's likely what happened to you.
If you try again, you should be able to sign up immediately (and if Facebook fails again, you'll see an error screen rather than the confusing thank you screen). Sorry for the inconvenience.
Maybe after I've seen the product and am ramping up my use, this would be ok, but before I've seen it? Poor form.