Her Facebook public figure page has had more than 96,000 likes.
Clark told Newstalk ZB she had previously "hovered" around fifth or sixth place in the rankings.
"I think the key to it is that I actually do do my own tweets and eventually that picks up a following - it's authentic and people like it."
Clark said that when she started at the UNDP its communication policy was "inward-looking" and so she set out to use social media to promote the work the agency was doing. She believed her legacy in that role was that the UNDP now communicated its work much better.
Even though she was now a "free agent", she still believed it was important to take the time to send a tweet or stop briefly to take a photo to share.
"I take a lot of interest in what's going on around me I offer opinions on it."
Clark is ranked at number three, behind Hillary Clinton and Harry Potter author J.K. Rowling.
The report was compiled by Richtopia.com, a digital periodical that covers business, economics, and financial news.
Clark is ranked above TV star Oprah Winfrey (5th), former US first lady Michelle Obama (6th), British Prime Minister Theresa May (10th) Ivanka Trump (16th) and Yoko Ono (26th).
Richtopia uses an algorithm, based on social media influence, to determine the ranking order of 250 women.
"We take into account various metrics from Twitter, Facebook, Wikipedia, Youtube, LinkedIn and Instagram," the digital publisher says.