Before the advent of email, senders bore the brunt of communication costs. Spam is an economic problem, and solutions with the greatest potential are seeking to correct this imbalance. This is well known.
But consider IM for a moment. Yet another Push medium, the most efficent way to get someone's attention happens to be very expensive for others. Not only for the time you are interrupted, but the interruption tax of 15 minutes it takes to cognitively recover from the task at hand. Receivers are responsible for communicating presence to avoid interruptions, but we don't have ways of automagically signaling presence that is both rich enough and leverages the social network as a filter. Heck, the most efficient ways of communicating rich presence is asynchronous (blog posts, Flickr, Plazes) and yet to be integrated -- there is no Xfire for real worlds.
When you factor in the rise of RSS as a Pull mechanism that the receiver controls -- there is a significant shift underway to make senders pay. If you don't write a worthwhile blog post, people don't pay attention. Readers slap through posts with their space bar and have their trigger finger on the unsubscribe button.
Within the next five years or so senders will pay the postage due.
As social networking becomes core infrastructure, you gain the filter to respect privacy while enabling presence. Breadcrumbs will sprinkle trails beyond the beaten path of on/off/sleep. With cameraphones we are really just experiencing the first wave of rich and convenient presence. Presence that provides object-centered sociality to tell even richer stories.
The behavior we are seeing around events are prefect examples of what happens when you add Where to the presence mix. Today events provide a fixed object for activity to organize around and are public enough to share stories and artifacts without breaking social norms. When cell phones capture and constantly transmit spatial presence we may be in for the biggest privacy shock of our time. Like a camera over our shoulder, only it's in your pocket, everywhere and nearly always on. Social norms will significantly evolve.
However, with the social network as a filter -- coordinates of time, space and activity (what am I listening to, my calendar, use of modalities) can automagically provide a reasonably rich presence. When the cost of presence and interruptions are reduced from the receiver, we may find it more efficient to connect.