Ask HN: What makes a company worth staying with as engineer?

My impression is that many software engineers stay at a company for 1.5-3 years and then go elsewhere. It's usually at least financially motivated. I also read about people looking for new engineering challenges and responsibilities--specifically not management.

It seems like Google and Facebook have this solved. Are there any small or medium sized companies that typically retain engineers for more than 3 years? What do they do to make this happen?