Since DDG uses results from other search engines, and one of their main selling points is the "unbiased, unfiltered search results", I fail to see what Facebook would be buying in that case.
DDG has its own index, you get offered bing/google when there is too few results from DDG. Even when you click bing/google, it is anonymized, so the results are not skewed by the info the search engines have on you.