Thanks Mike. Great points and questions.
Marketing solutions (their content-monetization product) is 25% of revenue, whereas recruiting is 55% and premium accounts (which I suspect is a blend of recruiting/marketing) is the remainder. See here: http://press.linkedin.com/about
Remember that people in the sales & marketing community are not the typical use case for the other 277 million members on LinkedIn. Most people don't produce content; they all need jobs though. As a smell test, ask people in your family who have different careers how they use LinkedIn and with what frequency. You can also quickly still see how professional content is being shared. Go to any professional content site (like Social Media Today) and look at the social shares widget. Typically, it's Twitter in the lead, followed by Facebook, LinkedIn or GooglePlus depending on your vertical.
Finally, I hope this post doesn't suggest I'm "anti-LinkedIn." I use it daily, I have friends who work there (at somewhat senior levels) and I think it's a wonderful product. But I know a land grab when I see one and the move towards content publishing is not, in my humble opinion, a strategic move to improve content quality. Rather, it's a move to increase engagement on the site. If I'm a publisher working with LinkedIn, I would tread carefully.