"Hey bossman, my great friend XXX, who I worked with at YYY, is getting bored over there. He's a great programmer and would be a great fit for our team and the infrastructure position we have open."
And bossman will definitely give him an interview and massive bonus points for the personal recommendation and relationship, and wisely make him a better offer than some stranger that comes in from StackOverflow or GitHub.
This cuts both ways.
As an engineer, you want a job that a colleague recommends to you, far before some random posting you see on StackOverflow Careers.