Thanks everyone for your responses. I did go through the flow myself, and the Original Referrer says "google.com" for my linkedin campaign. It also says "google.com" when I downloaded it from a Facebook post.
So I'm leveraging the same form for multiple channels (linkedin, facebook, also on website). I thought marketo would tell me where ppl clicked from, but that doesn't seem to be the case.
How would I create a Hidden Field on my form to indicate the lead source? How does this actually work? Or should I be creating a different form for each channel?
Thank you!