I wrote the original version of this for the Atlanta Film Festival blog:
As a filmmaker, you probably already know that your film needs a web site. And if you don't, then hear this: you do. (Yes, even for short films, though a sub-page on your main company or personal web site will probably do for a short.) A Facebook page is not enough. A web site serves as the ambassador for your film to festivals, press, your audience, and the film industry.
There is now an expanded PDF ebook version of this article with examples and images -- and you can download it for free.