The Publicis Groupe-owned media agency has boosted the forecast it issued in December for 2012 global internet adspend, by 4.5%.
The sudden rise of social is evident in a revision to its US forecasts, which had pegged social media spend for 2012 at $1.33bn (£851m) in December, but now predicts $3.42bn (£2.2bn).
Jonathan Barnard, head of forecasting at ZenithOptimedia, said this was partly due to $1bn (£640m) of display spend being recategorised as social media spend, and partly due to the medium growing faster than previously thought.
Barnard told Media Week: "Social is justifying its hype now. A third of all display impressions in the US are social. Advertisers are learning to be less interruptive."
He added the revised perspective of how much social media was growing and contributing to larger markets had a knock-on effect on the forecast for 2012 overall internet adspend across the world, which is up from $84.3bn (£53.9bn) in December to $88.1bn (£56.3bn). (The agency does not give a breakdown for social media in its global forecast.)
In its review of the US, the agency comments: "Facebook, Twitter and YouTube are all building platforms that are attractive to small businesses, in addition to Fortune 500 companies.
"Social media is creating a new type of advertiser that did not advertise before, including individuals and content creators.
"Social media ads should also be expected to increase in price, based on current patterns, the influx of new advertisers, new creative formats, and improved targeting."
Zenith has also nudged up its forecast for 2012 global adspend growth across all media after turning more optimistic about Latin America, although it downgraded Western Europe.
Follow Daniel Farey-Jones on Twitter @danfareyjonesLeague Tables