Consider advertising and the monies required to do local, regional and national spots. Then you have to consider time slots to target your potential customer base.
Now with Facebook and Twitter, your message is in front of ~200M people (potential customers) waiting for them, instead on you taking a chance that they were watching TV or listening to the radio at a particular time.
Twitter pushes out "tweets" to you, so when you tweet a message, the users get them almost instantly. Here's an example of a tweet I got this morning from Engadget: