SAN FRANCISCO — As the U.S. Department of Justice tries to thwart the $11 billion merger of U.S. Airways and American Airlines, the Antitrust Division's unusual fight to unravel the tie-up of two little-known social commerce companies is turning heads in Silicon Valley.

A bench trial is set for Sept. 23 before U.S. District Judge William Orrick in San Francisco to resolve the fate of Bazaarvoice's 2012 acquisition of its main competitor, PowerReviews, which the government claims snuffed out competition in the market for online product reviews and ratings.