I'm extremely wary of posting original creative works online. My understanding is that once it's posted on the Internet, without some explicit statement of copyright ownership ©, it's considered "published" and it's public domain, or the property of the website to which it was uploaded. But I'm no lawyer.
Then again, I'm so paranoid, I have anxieties about emailing someone a copy of a manuscript, lest it get leaked and become commercially useless to me.