I lead a team of 4 PHP developers and we have been tasked with developing a custom social network site/portal. By social I mean Facebook-like social - activity streams, comments, popularity, various kinds of content, etc. 
We don't have years to develop it from scratch so I'm thinking about building on top of the existing CMS - either Wordpress with BuddyPress or Drupal.
The choice between the two is a tough one for me. My team is very proficient in PHP but has no significant experience with neither of the CMSes.
Both platforms provide similar end-user features but I wonder how easy it is to hack BuddyPress (if we need to, which is very likely) vs something (likely Commons modules) on Drupal.

The problem with Drupal is that D7 is there but Commons modules haven't been portedto it yet. Developing on D6 doesn't make a lot of sense to me as I've read it will be problematic to migrate to D7 in the future.

The social network site we have to develop is not really centered around a blog. What would you recommend?