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As part of the German carmaker’s promotional effort for its clean diesel engines, Audi will donate $1 to the Nature Conservancy’s carbon offset program for every Facebook user who joins the program, up to $25,000.

Audi TDI clean diesels reduce carbon dioxide emissions by up to 20% and increase fuel efficiency by up to 30% over gasoline-powered cars, according to the company. “Audi’s contribution will provide necessary support to our voluntary...