Google has 'social' platform already - Gmail. However, what they have tried to do so far is to force 'social' elements into their current offerings, and they have tried to append external turn-key systems like orkut.
You can't force social behavior onto your users, as they have successfully demonstrated. As I see it, only two successful platforms so far, facebook and myspace (which died) had a different start. They started out from human element, which is people started banding together and dragging along their friends into it - onto a central place. Email is social too, but it isn't one central place, but more peer to peer.
What I think Google should do is to do something they have never done before. Humanize Google. Introduce customer support, active engagement with current user base, expand into traditional media, and not only with google ads, but with interactive shows based on google platform, editorialized part of google news, etc... only then will users start to feel google is something they can empathize with and concentrate around it vs only using their services and go away.
tl;dr; humanize google, bring human element to it. Actual humans into their services.