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.

yep , this. and even twitter is somewhere-in-between

The idea is that you didn't hear everyone, or even a significant amount of people, by default. You effectively had to find what you wanted and make efforts (joining chats, RSS feeds, etc) to tune-in. Or... make lists of things to... unblock.