A friend of mine, Nishi Serrano, a featured Horror author in the Hellfire Book of Beltane Anthology (yes, that is a shameless plug for her frightening tale “Old Looshi” in that Anthology) on occasion puts up an excerpt of her work in progress or a segment of already published material in her Blog. When I asked her if I should do this, too, her answer was a resounding, “NO!”
She explained that the segment she crafted utilized a character for whom she already had copyright protections in place or she wouldn’t have done it.
Putting up segments of your own already copyrighted work, is fine, as long as the publishing house you are with, you and your agent all agree on what limitations, if any should apply.
Use caution with your work, and make sure that if you put it on a public site that you own the copyright for your characters. But be aware ,too, that some publishing houses won’t touch work that already has a copyright, including those published on a Blog or Facebook, Twitter, etc., by their own Authors. The legalities and risks are changing somewhat with e-publishing business coming rapidly to the fore, but again, use caution and do a bit of research to protect your writing and your interest of same, to do otherwise could be a scary experience fit for a horror novel!