[jasonwatkinspdx]

Another key difference is that email users generally contribute directly to their provider's infrastructure costs in providing email as a service. Email infrastructure (and the user experience) is fragmented, and global funding generally scales with global load.

Instead twitter must monetize via advertising of some form, and so the percentage of folks who do not respond to ads acts as a really strong factor in your cost calculations. In this sense, email software has it easy, and can be extremely wasteful in the resources it consumes.

It's not just that the availability expectations of twitter are higher than email, it's also that the economic base of the infrastructure is far more sparse.

[InclinedPlane]

And yet another key difference is that there are few email server installations that support half a billion users. Saying that the scaling problem is "solved" because all you have to do is copy, say, gmail, is kind of silly.