This is in fact an unsolved problem. Current database systems cannot handle this level of connectivity. Facebook can't do a "push" because of all the features t

This is in fact an unsolved problem. Current database systems cannot handle this level of connectivity. Facebook can't do a "push" because of all the features they support. It gets extremely complicated if they push complex data into user's mailbox. And twitter cannot do "pull" so they can avoid putting a cap on the number friends/followers but then that's why they cannot really add any feature.