Facebook Inc.’s (Nasdaq: FB) 2013 earnings will be “significantly below” analysts’ estimates, said Richard Greenfield, an analyst at BTIG Research, citing the social- networking site’s struggles to get revenue from mobile users.

“We’ve been growing increasingly skeptical of some of their monetization methods,” Greenfield, who cut his rating on the shares to sell, said in an interview today on “Bloomberg Surveillance” with Tom Keene.

Facebook’s payments business is “collapsing” as companies such as game maker Zynga Inc. grapple with lower demand, putting more pressure on Facebook to increase advertising sales, Greenfield said. The social-networking company risks annoying its mobile customers by cluttering the service with ads, the analyst said.

Shares of the Menlo Park, California-based company declined after Greenfield changed his recommendation from neutral. The analyst lowered his 2012 and 2013 revenue forecasts, as well as his estimate for earnings before interest, taxes, depreciation and amortization to $3.05 billion. Analysts predict about $3.5 billion, the average of estimates compiled by Bloomberg.

Much of the performance of mobile ads is driven by accidental clicks, or “fat fingers,” according to Greenfield.

“It feels like Facebook is pushing advertising monetization harder than they should be, which we believe will harm user engagement in 2013 and beyond,” Greenfield, who has a target price of $16 on Facebook shares, wrote in a report.

Facebook fell 1.4 percent to $20.61 at 10:38 a.m. in New York, after declining as much as 3.6 percent to the lowest intraday level in about a week. Before today, the shares had slumped 45 percent since the company’s initial public offering in May.

JPMorgan Chase & Co. and Morgan Stanley cut their price targets for Facebook last week, citing lower expectations for revenue from the site’s payments business after Zynga cut forecasts.