It has nothing to do with the nature of the activity. Graham's point is that the social networks provided the last stimulus needed (over the last 30 years) to get computers into daily use in almost every household in the first world.
You didn't cancel your cable, but you likely spent more time in front of the computer after picking up AIM and FB than before. This signal indicates the front edge of a trend. This is visionary stuff, not hindsight.
