So you think you're talking to the WORLD when you post something? Well, you're not...but you could be...but really, you're not. So who is listening?!
Think of a social media site as a giant hotel. You just entered the hotel and checked in. Now you have a "room."
You invite some friends to your "room" and they stop in, indefinitely. Now all of your friends are gathered in your "room" and they listen to what you have to say (for the most part) and occaisionally give you a "nod" with a "thumbs up" or a "like" or other positive marking to represent that they approve of your comment.
So, each "room" has countless closed "windows" into other "rooms" where you can see others talking to their friends. You may like what you see and invite them to "share" their conversations with you. When you begin to "share" with others, your "windows" become open doorways that allow your friends to wander in and out.
Now picture yourself talking to friends and one of them invites a friend in to your "room." They say to their friend, "here's someone you should visit." You must approve that they are allowed in to this "room" and that you are ok with it.
Now, when they visit, your words travel through them to their timeline and are shared with a bunch of their friends, who may in turn, friend you as well.
Having said all of this, you can see how social media CAN spread if you are saying things that people like to share with others.
Here's where reality sets in.
When you post a comment, or photo, or video, it is ONLY seen by SOME within your friend group ("room") and may never travel beyond that. So ranting to the idiot that just cut you off in rush-hour does absolutely nothing because you are actually yelling at JUST your own friends! Keep that in mind as you post!
On the other hand, if you want to offer something special and exclusive, you might have your message shared, retweeted, posted, pinned, or pushed all over the World!
Summary? You are only talking to those that care, or care to share!