Why DuckduckGo improve so fast? - I can say they're running a privacy focused search engines. I think this is strong enough to convince internet users to prefer using it instead of using other search engine, and 90 percent of Internet users re concerned about their privacy. However, some of the growth could be attributed to the popularity of the DuckduckGo Mobile Application.
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However, inspite its rapid growth and expansion, DuckDuckGo still has a long way to go before leveling up to Google. According to a survey, Google has 87.33% of search engine traffic in the US, and DuckDuckGo has only 2.53%. DuckDuckGo, on the other hand, is swiftly catching up to Yahoo, which accounts for 3.3% of search traffic. 6.43% of search traffic goes to Bing.
DuckduckGo has recently launched a desktop browser, however, this could help the company gain more users and expand in search market shares. A user who download the browser will possibly use the private focused search engine because the engine will be automatically install to the browser unless you change it to whatever you prefer.