Should you be concerned about your intellectual property being stolen when you post to Facebook?
I've read from other people that Facebook has in it's terms of service that they own or at least may use any content you post on Facebook. That may or may not be true. I have't read it for myself on Facebook's Terms of Service (TOS).
In terms of other people stealing your material it's probably going to happen anyway. However consider that if you only put up a short part of your story, readers will be compelled to want to read more, therefore it will be great to have a link going to your own site, blog or well your book may be bought ie Amazon.
One of the best things you can do at least in the U.S. is to register your copyright with the U.S. Library of Congress that way if someone does steal your intellectual property you have an abundance of legal recourse under the letter of the law. For the record I'm not an attorney and this isn't legal advice. You should always seek the advice of an attorney when considering your legal position, etc. :-)
From a marketing perspective, consider that if you don't have a million dollar budget to promote your book it's not such a bad thing if people use and repost your material, as they are promoting you for free, so to speak. I too write and publish books, so I have a concern about the use of my material. Then I remember all the people who can't give their work away. To have someone wanting to to steal your material isn't all bad when considering the possibilities.
How to Self Publish for Profit