The short answer to the "When should you use paid social?" for small business is - if you've established that organic social traffic to your site has proved a positive ROI and it's of value. Then, you can use those learnings (audience interests, targeting, geo areas, etc) to use paid to push to similar and look alike audiences, ideally reaching new users who fit your consumer profile on the platforms (FB, IG, Pinterest, etc).
If your site doesn't currently have social, or it's got no social traffic from existing profiles then your first step isn't spend, it's figuring out with organic posts what content works for you.
