Google and Twitter's engineers are hard at work integrating Google's search results with Tweets in real-time by the first half of 2015. Twitter has similar arrangements with Bing and Yahoo, and will receive an unspecified amount of data-licensing revenue.
Twitter had this arrangement with Google from 2009 to 2011. This renewed relationship with Twitter eliminates Google search shortfalls relative to Bing and Yahoo. It also helps Google reduce the indirect threat posed by Twitter as a real-time newsfeed.
Facebook hides data from Google, and has privacy settings and restrictions on data sharing. So, it's typical for Facebook posts never to be crawled or indexed by Google at all.
A goal for Google+ was using it to power real-time search after Twitter cut off Google's access to its firehose in 2011. Since then, Google has been using Google+ to discover new content, and inbound marketers have made it a best practice to share links on Google+ as they're crawled and indexed immediately.