But it is nearly impossible to systematically keep track of the candidates that aren't hired to see if they are successful elsewhere. LinkedIn could offer this

But it is nearly impossible to systematically keep track of the candidates that aren't hired to see if they are successful elsewhere. LinkedIn could offer this as a service, if there was sufficient demand that they could monetise it. Every company will routinely check out every candidate’s profile anyway. They just need an automated way to check again in 6 months and see if everyone they reject is going to a competitor who is beating them in the market.