WP forum plugin

[cprpoker]

WordPress MU › Blog Tool and Publishing Platform - run as many blogs as your hosting will allow from one install.

buddypress was built to turn the wpmu into a social network site. (not meant for wp)

Exactly.

There is a Beta of BuddyPress that allows you to run it in WP:

BuddyPress.org – WordPress Social Network Software → BuddyPress 1.2 Beta

When WP 3.0 releases (sometime in March?) this will release from beta shortly after.