Google's Flight Search feature was launched in September 2011. Though Google hasn't shared the exact number of routes added but have hinted that Flight Search has expanded to almost double its size since inception.
Previously, this feature was limited to big cities around the world. Google's Flight Search, now provides information about domestic flight routes in the US including domestic airports, of lesser known cities, through which they pass. With so much achieved, we still have numerous flight routes that haven't made the cut yet. We are assuming it's only matter of time, before all the domestic US routes and domestic airports are made available on Flight Search feature.