scaling wordpress for lots of users


We have a general question on WordPress vs Drupal when it comes to sociability and customization. We are working on a new site and if it generates thousands or potentially millions of users over time, is WordPress able to handle it or can it be optimized to handle a large number of users? The site is running multisite and there could be users of the main site as well as sub sites under it.

Further, we would like to use Buddypress or some social tool as part of the sites. Buddypress currently is a bit difficult to work with and play with themes. I understand there may be some changes coming there to make it work better with any theme. Do you have any insight into when that change might be coming or tips on customizing themes to work with Buddypress better.

Our site is going to need some customization and just want to make sure we pick the best platform to start.