Capital Research & Management wanted to buy into the Facebook Inc. initial public offering. But days before the IPO, an underwriting bank on the deal warned the big investment firm about Facebook's dimming revenue prospects.

The Los Angeles firm, armed with information from a May 11 "roadshow" meeting with underwriters and Facebook, along with similar estimates of its own, slashed the number of shares it intended to buy. The night before trading began, a Capital Research manager...