First, as a quick snapshot of our recent growth, in the fourth quarter of 2021, we delivered $106 million of revenue and 38% adjusted EBITDA margins, ending a year of exceptionally strong execution on a high note.
In 2021, we measured 4.5 trillion ad impressions, resulting in record revenue of $333 million, an increase of 36% compared with a year ago.
We grew faster than the digital advertising industry and significantly outperformed the industry's programmatic, social, and CTV growth trajectories, while generating 33% adjusted EBITDA margins.
ABS revenue grew 77% in 2021 and contributed approximately $85 million to our top line.
While a significant majority of our top 100 clients are now using ABS in some applications, their usage of the product tends to be in North America where programmatic buying is dominant.
In the fourth quarter, for example, we activated ABS with Disney in Latin America, Colgate in EMEA, and Nike in 21 global markets.
Beyond our top 100 customers, approximately 40% of our top 500 clients still do not use ABS in any market, representing a solid expansion opportunity among this established customer base.
We expect this unique differentiator of DV's programmatic solutions, including ABS and DV custom contextual to further drive our programmatic sales momentum and support DV's overall RFP win rate, which was 80% in 2021.
Our go-to-market strategy for authentic attention leverages DV's established customer base of over 1,000 leading brands to drive ubiquitous uptake of these new data sets.
Social revenue grew by nearly 50% in 2021 with strong performance across Facebook, YouTube, Twitter, Snapchat, and Pinterest as more than 300 new advertisers activated DV's social media verification solutions last year.
Beginning with TikTok, we continue to expand coverage of our viewability solution, which is now available in 67 countries.
Our solution has been used by nearly 30 advertisers, resulting in average monetized impression growth of approximately 220% over the last six months.
On brand safety and TikTok, DV's advertiser activated brand safety controls continue to expand and have now been rolled out in North America, the U.K., Australia and the Middle East, with 82 advertisers using the solution.
On YouTube, we saw that when our pre-campaign activation solutions are applied to campaigns, brand suitability incidents are reduced by up to 50%.
On CTV, our impression volumes grew 57% in 2021.
And by the fourth quarter, 25% of our tag-based advertiser video impressions were CTV.
DV customers are fully protected from this scheme, which continues to scoop more than 5 million devices and over 80 million ad requests per day, undercutting ad investments and underlining performance.
To combat this latest viewability challenge, last month, DV launched fully on-screen prebid targeting, enabling connected advertisers to target inventory from sources that are tested and evaluated by DV to ensure ads are only displayed 100% on screen and when the TV screen is turned on.
International revenue grew by almost 70% last year with APAC revenue growing by 84% and EMEA by 61%, all outpacing the industry and our competitors.
International now contributes 26% of our overall direct revenue.
We currently generate revenue in 93 countries and from our expanding base of 20 locations outside the United States, we will leverage our exceptional RFP win rate to take advantage of the expansion opportunity that exists in markets around the world.
In 2021, 55% of our headcount growth was driven by international hires as we continue to invest in expanding our global presence.
We signed 176 new advertising customers in 2021, including brands such as Target, GEICO, Diageo, BMW, Bumble and Apple services.
61% of our new logo wins were greenfield, while 39% were competitive wins.
In addition, on the supply side of the business, we added numerous new platform clients such as Amazon, Taboola, AdTheorent, Smartclip, and Verve, as well as 19 new publishers to the fold.
In the last 12 months, DV has launched or expanded the only widely available attention solution, the only comprehensively accredited programmatic suite, the only solution for measuring and filtering fully on-screen CTV impressions, the only certified CTV fraud program for programmatic partners, and we are the only leading verification company to root out and publicize the numerous new fraud attacks that shake the confidence of digital advertisers around the globe.
We generated $106 million of revenue, representing year-over-year growth of $27 million or 34%.
We grew fourth quarter adjusted EBITDA to $40 million or 46% year over year, representing a 38% adjusted EBITDA margin.
And as previously mentioned, we anticipate the integration of OpenSlate solution to generate between $15 million and $18 million in 2022.
For the full year 2021, we delivered $333 million in revenue, up 36% year over year and adjusted EBITDA of $110 million, up 50% year over year and representing a 33% adjusted EBITDA margin.
In 2021, advertiser programmatic grew 45%, driven by ABS, which grew 77% and represented 50% of advertiser programmatic revenue.
Advertiser direct revenue grew 27%, driven by social revenue growth of 47%.
Social represented 33% of our advertiser direct revenue, up from 29% in 2020.
Finally, supply side revenue grew 38% in 2021, driven by new deals with large platforms such as Yahoo!
Japan and Amazon, as well as the 19 new publishers we signed on during the year.
Our 2021 net revenue retention rate was 126% and while gross revenue retention was 98%.
Our customer tenure was 6.9 years for our top 75 customers.
For our top 100 customers, we grew average revenue per customer from $1.8 million in 2020 to $2.2 million in 2021.
And finally, we increased the number of customers generating more than $1 million in revenue by 42% in 2021.
Our cost of revenue increased by $19 million year-on-year in 2021, primarily due to an increase in costs from revenue sharing arrangements with our Programmatic partners as Programmatic revenue grew as a percentage of total revenue.
In 2021, we expanded adjusted EBITDA margins to 33% while continuing to invest in the business.
We added over 200 employees during 2021, including approximately 100 from our two acquisitions.
In terms of cash flow and balance sheet, we generated $83 million in cash from operating activities in 2021, a nearly fourfold increase from the $21 million generated in 2020.
We had approximately $222 million of cash at the end of the year and zero debt on the balance sheet.
We expect full year 2022 revenue in the range of $429 million to $437 million, a year-over-year increase of 30% at the midpoint.
And we expect full year 2022 adjusted EBITDA in the range of $126 million to $134 million, representing a 30% adjusted EBITDA margin at the midpoint.
For the first quarter of 2022, we expect revenue in the range of $89 million to $91 million, which implies a 33% growth at the midpoint.
And we expect first quarter adjusted EBITDA in the range of $21 million to $23 million, which represents a 24% adjusted EBITDA margin at the midpoint.
Stock-based compensation expense for the first quarter of 2022 is expected in the range of $9 million to $10 million.
For the full year, stock-based compensation expense is expected in the range of $44 million to $49 million and shares outstanding for the first quarter are expected in the range of 170 million to 173 million.

Summary:
We generated $106 million of revenue, representing year-over-year growth of $27 million or 34%.
We expect full year 2022 revenue in the range of $429 million to $437 million, a year-over-year increase of 30% at the midpoint.
And we expect full year 2022 adjusted EBITDA in the range of $126 million to $134 million, representing a 30% adjusted EBITDA margin at the midpoint.
For the first quarter of 2022, we expect revenue in the range of $89 million to $91 million, which implies a 33% growth at the midpoint.
And we expect first quarter adjusted EBITDA in the range of $21 million to $23 million, which represents a 24% adjusted EBITDA margin at the midpoint.