It is really challenging the monopoly of the giants like Google & Bing by not tracking you.
What can challenge social networks like Facebook & Twitter?
And mainstream users don't really see the problem with giants or tracking, they see how Google's search results page is getting more and more complicated with features they don't need. The original reason to use Google instead of, for example, Yahoo!, was twofold: the website looked very clean and the results were good. Now DDG's results are nearly matching Google, and they provide a much better interface plus a few features (mostly for developers, like MDN integration). What happens? People switch.
If Facebook works good for users, I guess they'll stick.