Although it may appear that people share status updates with everyone on their network, they often have different audiences in mind, and most status updates are intended for specific individuals and groups. We can think about four audiences:
The update is explicitly directed at specific people. No-one else would understand the update.
There is a specific audience in mind that is implied from the content, for example asking whether people want to go to an event together. It’s easier to post the update to everyone than to try and send it to just those specific people.
People have a specific audience in mind, but don’t consciously think about it before typing and hitting ‘publish’.
Sometimes people have an explicit audience in mind, but broadcast the update to everyone. This is because the benefit of receiving a serendipitous reply outweighs the fact that they might send updates to people who don’t care about them.