What’s Missing From This List?
Groups. All three of them have groups has major functionality, and none of the three have added groups as a component of the top or main navigation. Since all three of the services need advertising and page views to a certain extent, you would think each would try to drive traffic to groups.
My opinion? I think groups are underused; with the right approach, groups could be mini social networks upon themselves. For the services above, maybe they haven’t gotten there. But, if they could increase traffic in highly targeted and focused groups, why don’t they?