Thanks for your comment Andrew! Yes, I find it quite suprising too.
Some of those with inactive/incomplete profiles are quite successful and very networked, but not online. It's not hurting them now, at least not in obvious ways to them. But for the ones in their 40s who are still likely to have many productive working years left, it may.
Most employers looking to hire sales, marketing, communications,customer service, managerial, key leadership and other roles are, I would guess, wanting this skill and knowledge in potential candidates. Even when the person doing the hiring ISN'T knowledgeable in social media themselves.
Even now and in the future, where there are two candidates vying for the same job, one's experience and knowledge of social media could be the difference maker/deciding factor.