Google Chairman Eric Schmidt has said that they are not favoring Google Plus over other social networks. But they need the relevant permissions to access content from user profiles to index them.
Google already index publicly available content not blocked through the robots files using their search spiders but this is not the same thing as having access to the content directly.
Twitter for example chose to not renew their deal with Google which let them access their entire tweets database. Funny enough, Twitter itself does not have the capacity to let people search through their own tweets from the very beginning. Facebook incidentally too does not allow you to search your own posts. Google is offering this ability on Google Plus and has made social search more useful by integrating it with their search engine.
Schmidt essentially said that they would be more than happy to integrate content from Facebook and Twitter on the Google Social Search engine and they are very interested in talking to these companies for the same.