I use two fields: lead source and lead source detail. For lead source, I put the event or big picture item/tag that they're participating in. Lead source detail is where they came from. So, for instance, lead source would be webinar, lead source detail would be facebook ad, or wherever they came from.
Although this doesn't capture all interactions, we choose to measure where they came from, then analyze ROI from that campaign in particular. However, our sales cycle is faster than most.