Scalability of WordPress

I have a general question. We are working on a community site that will be part personal, part social where users will post content back into the site using buddypress. There is a potential for the site to generate millions of users a month, especially if we can secure a large retailer as a partner (in the works).

What is the realistic potential to use wordpress/buddypress to operating such a social netwok? I know Edublog has a lot of users and I assume they are pushing content back into the site vs just reading…but not sure if they are all on the same network either.

I like the tools, ease of use and some of the out of the box stuff of WordPress but if we know it will not scale to meet the potential down the road, should we work to change it soon to another more customized platform.

Is it about optimizing/separating the database up, etc, or ???