Re:i wonder what they mean by.. (Score 2, Informative)

This post, from the author of a similar system, breaks down the pros and cons of an email-like approach to the problem.

One simple but painfully restrictive solution is to duplicate the data for each user. Basically what this means is turning the service into an email system. Each user is given a mailbox and whenever someone they are following publishes a status, it is copied into their inbox, as well as into all the other followers' inboxes. This brings the solution in line with existing systems such as webmail services where data partitioning alone can accomplish great scaling. But this comes at a price. First the data storage grows significantly faster and requires much more disk space, together with increased backup cost. Second, it makes other actions much more complex such as erasing a message once sent.