> I don't think pushing tweets to every user is a scalable approach. It would seem more sensible to read rather than push tweets associated with a user. I don't know about Twitter's infrastructure, but storing more than 1 copy of the same data wouldn't make that much sense.

Suppose I have 5000 followers. How do you propose to get the timeline? Since you said "single copy of data", I assume you would do something like this:

https://github.com/mitsuhiko/flask/blob/master/examples/mini...

At twitter scale, that query will be a major bottleneck.