People change companies every few years. If a company has 100 engineers and half of them get new jobs after 3 years, you could now have people with 2nd-order co

People change companies every few years. If a company has 100 engineers and half of them get new jobs after 3 years, you could now have people with 2nd-order connections at 20-50 different companies. Repeat for several cycles and it becomes very easy to find someone who knows someone who worked with a candidate. LinkedIn makes this extremely easy.