I wasn't sure about LinkedIn, only because I'm not super active on it from a social perspective. However, this would totally be my experience in terms of engagement on social networks. It's kind of a no-brainer that people would stay on YouTube longer since it's a video based format. What may surprise some is that while Google+ is often called a Ghost Town (to which I'd say YOU'RE DOING IT WRONG), and does in fact drive less traffic… overall it's traffic is of a higher quality; second only to YouTube.
What do you think?