I think the issue here is that McLuhan was absolutely right when describing how the medium actually transforms and filters the messages that are sent through it. It's worth reading "Understanding Media" (or "The Medium is the Massage") because one of his key points is that the apparent content of the messages being sent across a medium is a distraction for what's really going on (the societal change brought about by the medium itself).

In the case of social networking the medium is broadcasting a message that it's normal to continuously stream updates, pictures etc. about your daily life as if the people following you cared about that. The fact that they do care about it is simply a sign that the medium is shaping society to the social network's particular medium.

Also, it's worth considering the difference between a one-to-one sharing of something (let's say a picture of your child shared with your mother) and the broadcasting of information to a chosen social group.

The former is a personal interaction which has many subtexts (proud parent showing off their child, wanting to connect with your own parent), whereas the broadcast does not have the same depth. It's just "look at me".