WhoKnows not only derives expertise, but it can also automatically derive and measure contacts and relationships to build your organization's social network. As an admin, you can determine whether you want your users to import their contacts or not.
Go to the Admin link in the account dropdown
Go to the Settings page and click the Who Knows Who tab
Click "Allow users to import email contacts" to enable/disable this feature.
Once you enable the feature, additional settings will become available.