Product-market fit is best measured by observing how customers react to your product. Better yet, observe them interact with your product. You know you’ve achieved product-market fit when the customers intuitively understand what need the product fills for them, and they have no trouble using it, in fact they enjoy using it… in fact they start telling their friends about it, maybe even telling the world about it on Twitter or other places. That’s how you know if you’ve got product-market fit.

A founder’s notebook is the best blog of all time.