I assume the commenter means that, with modern social networks, you specify a list of people with whom you socialize, instead of previous networks (perhaps like

I assume the commenter means that, with modern social networks, you specify a list of people with whom you socialize, instead of previous networks (perhaps like IRC) where you stopped by and could socialize with whoever happened to be there. Of course, this is not really the case. Twitter is a great example where the default is that anyone can stumble upon your profile and tweets.