treating a social network like a private club

The clue is in the name. It's supposed to be social. If you only ever interact with the people you know that's not being very sociable.

Would you go to a club/bar/whatever where there were many people you didn't know, but only talk to the ones you did? Even if there were some very interesting looking people there?

When using a social network, make your data searchable, you never know who you might meet.