Many writers I know post their work on a website or blog and are unaware of the risks. I've done it myself. Like many others, I found out the hard way that this may not be a good idea. Friends of mine have had paragraphs, openings, and even their entire novel ideas stolen. As horrible as it is, it happens all the time. But are the risks worth it? That is a topic of much debate lately.
What are we writers to do when we want to put something on our blogs and sites that will peak an agent or editors interest? Write a good book, edit it well, then right a good query and start sending it out. If they like your work, they may check your blog or website to get a better feel for you. But they won't be doing it to get a better feel for your work. Once your novel has sold to a publisher they'll help you decide what you should put on your site. At that point its protected and will soon be published anyway. Until then, it isn't needed and may only lead to frustration and heartache.
Not to say you can't or shouldn't put some of your writing on your site or blog, you absolutely can. Just be aware of the risks and post responsibly. The bottom line is, if you don't want it stolen, don't post it. I talk to my blog followers, Twitter followers, and critique friends about my book but I do so privately. We share information and critiques via e-mail or direct messages. Protect yourself and your work by sharing it only with those you trust. If you want to post bits of it do so knowing that you are giving it away in a sense.
Check out what a few industry professionals with Writer's Digest have to say on the matter:
Chuck Sambuchino: http://tinyurl.com/294tkq6
And an opposing opinion by Jane Friedman: http://tinyurl.com/25krwrf