Decentralizing social media s likely to become a hot topic.
Dave WIner has created RssCloud to enable more or less real time RSS updates and notifications, helping to decentralize the notification system. Twitter was an interesting model for a while, but it has demonstrated that it does not scale to a real flash mob. Sure it works well for large traffic volumes, but when there is a massive spike in traffic the centralized model is always going to be in danger of slowing down.
At some level high traffic is indistinguishable from a denial of service attack, sure the traffic is wanted, but if the servers cannot handle it, then the system exhibits the same behaviors that it would under a real denial of service attack - no new traffic gets through in a timely manner.