Enable BuddyPress Multiblog. You can put <em>define('BP_ENABLE_MULTIBLOG',true);</em> in your bp-custom.php or wp-config.php(recommended). For more information, please take a look at this http://codex.buddypress.org/bp_enable_multiblog/
Network Activate BuddyPress
Upload buddypress-multi-network to your wp-content/plugins directory
Visit NetworkAdmin->Plugins->Installed
Network activate BP Multi Network
Now you can visit the dashboard of any of your sub network(blogs) and you will see the notice to Install/Configure BuddyPress.
Follow those instructions to activate the BuddyPress settings on those sites.
Activate a BuddyPress Theme on your sub network and visit your new social network.
Oh yes, Now all of your networks(blogs) will have separate user base and separate social networks(Groups/Activities/Profile data). You can configure the Xprofile fields for a sub network from your Dashboard