Bing has hit 20 percent market share in the US according to new data from comScore. The slow and steady gains resulting in today’s milestone have come mostly at the expense of Search Alliance partner Yahoo. Indeed the two companies have effectively traded places.

Yahoo was last where Bing is today (20 percent) in May 2009. Bing launched in June of 2009. It still had a market share comparable to where Yahoo is today in late Q4 2010. Yahoo and Microsoft consummated their Search Alliance in July 2009.

Yahoo’s US search market share is currently 12.7 percent, boosted in recent months by its default search deal with Firefox. Google’s is down a tenth of a point from last month, at 64.4 percent. However search volumes at all the major engines saw growth.

The effective reach of Bing is nearly 33 percent, given that it serves organic results and ads to Yahoo search.