Google apparently doesn’t plan to let Facebook go unchallenged in next-generation commenting/discussion capabilities. Check the link below for details and a cute (albeit somewhat chaotic) video.
When we launched the new Google Docs last April, one of the big changes was moving comments to the sidebar and letting people reply to comments. Today, we’re updating comments in Google Docs to facilitate rapid and seamless discussions and integrate with email in an intuitive way. Since there are a number of significant improvements, this update is only available for newly created documents for now.