As with most web-browsers, Google was the default search engine in GNOME's Web, but instead they have decided to go with DuckDuckGo for GNOME 3.10 and future releases. DuckDuckGo is a partially open-source search engine that utilizes data from Wikipedia and partnerships with other search engines like Yandex, Yahoo!, Bing, and Wolfram Alpha. DuckDuckGo has risen in popularity recently since they claim not to track their users and have a better privacy record than the tier-one search engines.
Along with better privacy, GNOME decided to go with DuckDuckGo over better cooperation with GNOME, it's a platform that "works", and more. While it will be the default in GNOME 3.10, users can still opt to switch to other search engines if they wish.
More details on this search engine change can be found via this blog post.