Programmers in demand get approached by people about new projects. Demanding an NDA upfront starts the relationship off on the wrong foot.
Lawyers, paid to draft Non Disclosure Agreements, counsel their clients to make everyone they meet with about their new idea to sign an NDA. Programmers, vital to the modern business, get asked for advice all the time, but, as John Larson says, "will almost never sign an NDA."
Mixergy's Andrew Warner lists seven reasons explaining why programmers and consultants refuse to sign an NDA, including "Ideas are worthless," and "This reason is so secret that I can't reveal it … unless you sign an NDA." Starting a possible business relationship with a paper saying, "I don't trust you to keep our conversation private," sets an awkward vibe. Unless specific details, like private financial details, are involved, don't insult a programmer by waving an NDA as you start to explain your, "it's like Facebook but … " idea.
But keep in mind that sometimes an NDA is useful: You can use it to bash people who decide to break the honour code. maxklein on mixergy.com
Hollywood plot and character details matter, and are often under NDA. So are ideas in the patent pipeline. But your "It's like Facebook" idea doesn't need an NDA, because others are already working on your idea.