Scaling up Drupal

Hi,
I wonder, how much can I scale Drupal? How can it be done?
I mean, can it Support 50,000 users with 20,000 using it concurently. In that respect, can it build site like LinkedIn with milions of users?
If it does scale up as CMS can it also be used as development framework to build heavy proprietary applications that are used by such large number of usres?

Thanks,
Gili