Direct Response: Not So 'Direct' Anymore?

For example, I might receive a fundraising appeal but choose to go online (and not use the vanity URL) to give. Or I might receive an interesting mail appeal but forget to give — then I see a banner ad for the organization, which reminds me of its good work, and I choose to go to its website and make a gift. To what medium should you credit those gifts? These are just two simple examples of scenarios that are taking place every day as our industry becomes more innovative and interactive.