Specific terms of the deal were not disclosed.
PostRank launched in 2007 in Waterloo, Ontario, Canada. The PostRank staff will be moving to the San Francisco Bay Area to join the other Googlers at Google's main Mountain View campus.
Last year, PostRank Analytics added Activity Streams, which the company called "a FriendFeed for content."
The analytics service itself launched the year before as a way to track data on a large number of social media platforms, including Twitter, Digg, Delicious and more.
PostRank CTO and founder Ilya Grigorik wrote today on the startup's blog, "We are extremely excited to join Google. We believe there is simply no better company on the web today that both understands the value of the engagement data we have been focusing on, and has the platform and reach to bring its benefits to the untold millions of daily, active Internet users."