If we promote one webinar through paid ads, email blast, on our site, and social, what is the best way to report on this is sfdc?
No advanced reporting tools in this case, just Marketo and Salesforce.com.
there are several offer-channel methods posted here or on my site.
SFDC Campaign: Offer Name [Webinar]
Child SFDC Campaign: Channel Name
you will need a method to tag the URLs, etc. It's outlined on Marketing Attribution Setup Checklist - Marketing Rockstar Guides
The way I always deal with this is that the paid ad, social, etc is the lead source, and the webinar is the acquisition program. If you keep those concepts separate it becomes possible to see how many people "acquired" by the webinar came from different lead sources.
Did you try using different tags/statuses according to the channel?