TV reached a bigger audience but delivered only 60p of short-term sales for every £1 spent.
Online delivered 19% additional reach for the campaign, in particular reaching a higher proportion of younger consumers than TV.
Cadbury's 'Chocolate Charmer' campaign was tracked by the Media Efficiency Panel, a joint initiative between GfK NOP and Kantar Worldpanel that was seed-funded by Google.
Planned by PHD, the campaign ran last year from 9 April to 3 June and used TV, cinema, video-on-demand and online.
The data is the first to show a cross-media campaign's direct contribution to sales, according to PHD's media group manager Katrin Schlenzka.
Schlenzka said: "The TV plan was aimed at the wider Cadbury Dairy Milk audience whose heartland is older. Online was used to complement this and target the next generation of Cadbury Dairy Milk fans."
The whole campaign directly contributed to 3.5% of total sales, which compares well to other campaigns measured by the panel.
Sales were measured from 9 April to 14 June in approximately 7,500 households participating in the Kantar Worldpanel service, in which panellists scan and report their offline purchases when they return home from shopping.
The participating households' media consumption was measured by questionnaire, with the exception of online consumption, which was measured by software.
Video-on-demand exposure was not measured and there was no return on investment data for cinema.
Measured online activity measured included a YouTube home page takeover and a YouTube Promoted Video (which put the ad in Google search listings), exposure of the ad on Facebook, and a Google AdWords campaign.
The YouTube Promoted Video activity delivered 14.5% additional reach to TV alone.
The majority of people – nearly two out of three – who saw the ads online did not see them on TV.
A higher proportion of younger consumers saw the ad online (48%) than TV (46%). Online delivered two-thirds of the sales driven by advertising in this group.
Babita Earle, divisional director at GfK NOP, said: "We are extremely pleased with the results from this study. We believe there are some really key learnings on how digital platforms can be used alongside traditional media to really optimise the efficiency of campaigns."
Julie Reynolds, head of marketing for Cadbury Dairy Milk, said: "Cadbury are excited to be one of the pioneers of research connecting digital media investment with real-world purchases.
"This research gives us further reason to continue our investment in digital as a way to reach our consumers."