General Motors Co. plans to stop advertising with Facebook Inc. after deciding that paid ads on the site have little impact on consumers' car purchases, according to a GM official.

The move by GM, one of the largest advertisers in the U.S., puts a spotlight on an issue that many marketers have been raising: whether ads on Facebook help them sell more products. On Friday, Facebook is expected to sell shares in an initial public offering that could put a market value on the company of as much as $104 billion.

Executives...