Tracking the traffic sources of your leads will help you to optimize your campaigns!
You can do so in Opinion Stage by adding the following to your item links:
os_utm_source=facebook (This example is for Facebook)
os_utm_source=adwords (This example is for Google Ads)
You can then go to View Statistics and export your leads to a CSV or XLS file. Each lead in the report will contain the os_utm_source value.