Burning questions for ebook

I would have to agree it would be forums or FB. Or how about just Google it? Not sure what you mean by too public because if that's what people are talking about then that's what you'll want your eBook to be on right?

Forums and facebook groups will provide you with a LOT of information. Three questions of great importance are:

1. What information does this group want/need?
2. What information is currently available?
3. Why are people unhappy with this information?

For example

Niche: homeschooling
Current information: homeschooling tips and advice for US parents homeschooling for religious reasons.
Problem: people want information about homeschool in the UK or AUS. People want to homeschool but not for religous reasons. They need practical advice on the law, lesson structure, how to enter their children for exams, case studies of children in a similar situation.

Niche: weightloss | appetite suppressants
Current information: blogs filled with 500 word articles about how hard diets are and how appetite suppressants make things easier. 500 word articles basicially advertising one brand (made obvious by links to the sales page or a blog that blatently exists to sell this product).
Problem: people want to know if the pills actually work, what the side-effects are, whether better ones are available. They don't need to know why and how appetite suppressants can work or to read adverts telling them to buy a particular brand.

Niche: a particular technical product (movie editor for example)
Current information: articles giving the specs, more sales copy and basic information on movie editing in general.
Problem: people have a particular idea or project in mind. They want to know if this product will help them. Can they themselves get what they want from this product (which is a different question to is it possible). They want to see a selection of projects done using the product, step-by-step guides, etc.

Next step

When you have a good idea what people want - ask them to make sure what you're planning to offer them really is what they want. This can be open overtly or covertly on forums, in facebook groups on twitter and other social media.

Write a lengthly article 1000-2500 words on the subject area, tackling the problems you've discovered people need help with. Upload it to somewhere like Scribd and see the response you get. If you get healthy downloads and subscribers you're onto a winner. Now is the time to contact your outsourcer and get the big book written.

For a list of burning questions that can become chapters you can try yahoo questions and answers

Find a question that is asked multiple times but in a different way, have the writer write an ebook about it, and answer those yahoo questions with the link to your ebook

You can always go to amazon.com and look at the table of contents for the books that talk about a similar topic area to yours. Those chapter headings have been researched to be the ones that people want to know about, so you can't go wrong with those.