> With a second OS partition, the server can alternate between the working copy and a copy that is updated in a VM. For a free service, customers cannot complain even if there is a reboot every day and the service is down for a couple of minutes.

Are you seriously suggesting that a service (the size of Twitter, no less) has an acceptable downtime of a few minutes a day?

> Realistically, there would be a mirroring server to be prepared for hardware failures. One server can be restarted while the other is the main server.

But for that mirroring, you need to replicate disk writes, databases, backups etc.. This additional load would easily bring the server to a point where a single server would no longer suffice, even an insanely spec'ed one.