Mr Norman, who founded Dada in 2003, said the decision was made to capitalise on the higher growth opportunity offered by Itison.
The site, which continues to attract between 500 and 600 new users each day, is on course to turn over more than £20 million in its current financial year.
It booked pre-tax profits of nearly £1m on turnover in excess of £10m in its most recent accounts.
Eight members of staff have transferred from Dada to Itison, including Toni Morrison who now heads its events division. Dada, which employed 35 staff at its peak, will continue to operate as a "high-level marketing consultancy", however it will not work with clients on a day to day basis.
Mr Norman said: "The opportunity within Itison far outstrips the traditional model for a marketing agency. Dada [is] a profitable company. We had around 30-odd retained clients - the likes of Hilton and Domino's - and we can continue on with that. But the traditional PR agency model in which Dada was operating is very much a static model that isn't really developing, whereas [with] Itison we're forecasting revenues of £20m this year."
Mr Norman noted Dada has resigned most of its clients, with many moving to agencies recommended by his team.
He said: "It is what I would term more as a lifestyle business in many ways. It made a nice six-figure profit.
"More money-focused entrepreneurs might say "why on earth did you not sell it?" Those sort of businesses are just all about the people, and the most powerful asset in that was the people. If I take them into Itison the sky is the limit with the revenue opportunity."