Thinking out load, might be that big search engines are in cross-road trying to decide whether to make the search experience more social or more personal.
Facebook & Twitter may say: Search must Be more social! (friend-like, based on social signals search)
While others (Still shaking from the social networks power) altering with more 'personal touch' to the search.
What makes it almost impossible for new sites to enter and grow unless making high investment in social networks, branding and site's content quality etc.