I'd say: (almost) none of these.
GMail is a utility. Same with Earth, Maps (we're talking about social components for users, not for developers), Blogger (Tumblr, that is based on a social component!), Groups, Reader and Talk.
Social sharing in YouTube is extremely limited. I save my YouTube favourite videos on delicious, just to give you an idea, and never receive more than one suggestion a month about a video from the community.
Knol is a failure.
Orkut is strong only in Brazil (and India, perhaps) and is largely the (free) Brazilian AdultFriendFinder (you dive deep into it, and then you tell me).