US military recruiters operate as a referral chain through friends and family.

Evidence

That summer a marine recruiter called him at home. The recruiter had his number from two of Sami’s high-school friends who’d recently enlisted. This is one way that the U.S. armed forces fills its ranks, through a network that is essentially a big chain letter. The call caught Sami completely off guard, but it also intrigued him. The man was a professional and could sell the rust off an old bathtub. He talked a lot and promised Sami job skills, money for school, and travel around the world. — Moustafa Bayoumi, How Does It Feel to Be a Problem? Being…, 195