Consider paywalls and limited distribution posts (eg posts to a specific circle). These are just two methods of restricting content to a particular set of readers. The first is within the 'global search' tier, the second is in the 'social search' tier. Thus if you are searching for a specific keyword you are searching within publically accessible content and restricted content that you have a right to view.
Thus I'd claim that Google is introducing an unavoidable matrix into the search result world: horizontally, what access has the creator granted (global, social, personal) and vertically, what access has the viewer obtained (public, restricted, private). Of course, you can't show the user this matrix. But that's what users are coming to expect.
The former is something easily indexed today. The latter is where future standards need to arise: without knowing and unifying access credentials and authorization procedures and information exchange across web platforms (i.e. removing the walls between facebook, google, elsevir, etc) we will not be able to achieve a true unified and personalized search experience.
Reshared post from +Gregory Esau
is emerging as one of the go-to people for understanding our weaving webbed world.
He's so far ahead of me, I don't even know what words to describe what he understands and I don't.
Google Tiers Search into the Global, Social and Personal
By connecting Google+ to search, Google is tiering web search into three, very useful, layers: global search, social search and personal search.