I think if you read the terms of use that the member ratains ownership of his posts but grants SI a nonexclusive royalty-free license to use the contents. Thus SI could publish a book including your post and so could you without SI's permission.
From "Terms of Use" "3.2 You agree that upon posting information on the Service, you grant Silicon Investor, and its successors and assigns a non-exclusive, worldwide, royalty free, perpetual, non-revocable license under your copyrights or other intellectual property rights, if any, in such material to use, distribute, display, reproduce, and create derivative works from such material in any and all media, in any manner, whole or part, without any duty to account to you. You also grant Silicon Investor the right to authorize the downloading and printing of such material, or any portion thereof, by end users for their personal use." |