All you said was "Good read in my opinion. Keep it up." According to the rules of Fanfiction and Writing, all reviews have to be constructive. That is, not general. If you didn't want your post to be deleted, you should have said something specific about the story. What you did post was generic, could be posted on any other story and still make sense, and doesn't tell the writer that you actually read the story.
The rules are there to be followed. In order to avoid this happening again, read over them, and if you have any questions, ask.