You have a good point. I suspect, as sources of profiling data, Reader, Alerts, Wave, Buzz, etc. were not producing at the level of search and mail. Plus is to be seen, but it's already descending below the level of discourse on Facebook. (https://plus.google.com/+AstronomyPictureOfTheDay/posts – I guess that's the problem with integrating it with a bunch of cheap smartphones)