I am considering building a niche social network and I am concerned about Drupal's ability to scale when there are a lot of logged in users. I have read numerous posts saying that with caching, Drupal fairs pretty well for anonymous / non-logged in users, but that scalability is an issue with a lot of logged in users. The site I am developing would have to be able to handle between 200,000 and 1,000,000 logged in users a month. I could not find a lot of cases where Drupal is being used for a social network with a high amount of logged in users besides drupal.org. If you know of any, please list some below.

If you have experiences building social networking sites in Drupal, and have also done similar sites with a custom CMS, what would consider the ideal approach for developing this web application? Drupal or a custom CMS? What lessons have you learned in trying to scale Drupal for sites with a lot of logged in users? If custom, php, python or ruby?