This is a great application of the attention economy (/market) concept. With notifications as they are we are essentially giving away our attention to strangers for free, placing their messages with the same level of importance as those from our real friends. But an unsolicited message from a stranger can sometimes be genuinely important or valuable, so a system where notifications appear only if they are paid for, where the recipient sets the price, would eliminate spam while still allowing you to see what matters to you or what you are appropriately compensated for.