Before every meeting I have, I look the person up. If I use it, and most of my friends do, I can figure out who we know in common. If I need something from another company and I don't know who to contact, I look at 2nd degree connections for hints.
LinkedIn is in the toolkit of every tech-savvy business person. It can also be a people for engineers who want a good permanent place that non-technical people can find them. Non-technical folks won't look in Github.
Your last point is relevant. Yes, there is a correlation between LinkedIn proponents and Github profiles, but why wall yourself in to a Github only universe?