With Usabilla's Full Survey Campaigns, you can add your own custom variables using the URL.
These custom variables can be whatever you want them to be. You can, for instance, specify the source of your respondent.
1. Take the unique URL for the campaign of your choice.
http://survey.usabilla.com/live/s/51fa4e6706a1ea6b73000001
2. Add to this URL the variables you would like to collect, as specified in the example below.
http://survey.usabilla.com/live/s/51fa4e6706a1ea6b73000001?source=twitter
The custom variable can be anything you want, and you can add as many as you'd like - just add
?<Variable Name>=<Variable>
3. Once a survey is completed with these variables in the URL, the variables are carried to the results and stored in an extra column in the campaign results.