While I agree, I think that a large portion of developers want some way to set themselves apart from the masses - to be recognized as Uber 10X code ninjas. Those people will never rest till they create their own credentials, whether it be number of contributions to opensource projects on Github or Coderwall achievements or a standardized test.
That's because LinkedIn is not used by employees within companies to communicate like Facebook is used by students within a university to communicate. He's saying that there's a gap there that Google+ can fill.
You'd be surprised at the market for dog poop. It can be used as organic fertilizer, as a weapon, as a training aid (for training puppies). Seriously, I've purchased a bottle of dog pee on amazon for $7.99 with free shipping via prime. I know this sounds crazy, but I'm dead serious. I used the dog pee to teach my dog where he should go. But I wouldn't have known that was even possible without marketing.
Almost anything that you think is useless has a way that it can be used that justifies someone paying for it.