Promotion through any/every google product/feature (ie. authorship): they favour their own assets (YT, blog..)
Look how theyre managing to grow g+ even though it sucks and no one needs/wants another platform. Part of it is marketers using it because it is new, but also you can see that just like fb, apple, etc. 'dont be evil, but its ok to do evil' google wants to build its own walled garden...so you better operate in line with whats coming. thats the platform business i guess