Hi,
An additional URL parameter could be added with a custom plugin, since Affiliates uses the affiliate id to grant referrals to the right partner.
Perhaps you might be interested in Campaigns feature of the premium Affiliates Enterprise plugin. A campaign uses an additional unique URL parameter which helps you distinguish your statistics sources. Your affiliates will have the ability to use a campaign for Facebook, a campaign for Twitter etc and the plugin will keep track of each campaign’s performance. You can find more information about Campaigns feature in the Documentation page.
Kind regards,
George