Virtually all social networks, even when originally composed of people one actually knows, become "media" at about the point that any user reaches 150 people in the network, research suggests, because a human being cannot actually maintain more than 150 "strong link" relationships at a time.
Bruno Goncalves and researchers at Indiana University recently studied the network of links created by three million Twitter users over four years. These tweeters sent each a whopping 380 million tweets.
Researchers say it's not enough simply to follow or be followed by somebody for there to be a strong link. Instead, there has to be a conversation, an exchange of tweets. And these conversation have to be regular to be a sign of a significant social bond, so occasional contacts don't count.
At some point, "strong links" have to be replaced by "weak links." And turns any social network into a broadcast medium.