AFAIK all these - Bing, DDG, Yahoo, and more all compete in the same space. - are simply a single search engine (Bing) with some additions over the top. I suspect that there are about 4 true search engines remaining in the world, two of which are strictly regional (Baidu and Yandex) and Bing is only remaining afloat due to sheer willpower of MS which may end abruptly, as with some other products. I consider this state (with all other factors) as effectively a monopoly.