Scaling issues for social networks

"If you're willing to tolerate one disk seek, or if your graph has low fan-out (small number of people following any given person), you can de-normalize the data such that the metadata about every piece of activity is propagated to each of the followers of that activity at the time the action occurs. You might think of this as a "push" model. You'd still probably only store one copy of the actual activity data, but you'd push pointers to it (along with whatever other metadata is needed if you're supporting any ranking/filtering) to all the subscribers at the time it is created." - interesting reviews