I do not have a mail list of 500-1,000 people. I do belong to Facebook and linked in groups. Can I use those groups for product validation? Any other suggestions?

How big is your list at this time? 

We have see the product validation sequence work with >100 email lists if they are super targeted. 

You could use those communities for validation, but we would caution you to make sure you do it in a way that asks permission first. 

We recommend posting a question in your Facebook groups that says "I have a new project idea, I'd love your feedback! Leave a comment below and I can shoot some details your way".