Shares of Facebook (Nasdaq: FB), the No. 1 social networking site, fell 1 percent Thursday despite CEO Mark Zuckerberg’s announcing the site now claimed a billion members as of Sept. 14. The news came ahead of the company’s scheduled announcement of third-quarter results on Oct. 25.

In late morning trading, shares of the Menlo Park, Calif., website fell 18 cents  to $2165,  bringing the total loss to investors who bought shares at Facebook’s initial public offering to 43 percent.

Zuckerberg, 28, who’s been on a trip to Russia, announced the news before appearing on the “Today” show on Comcast’s (Nasdaq: CMCSA) NBC network. “This morning, there are more than one billion users using Facebook actively each month,” he wrote in an update to his personal page.

Facebook had previously said it had 955 million users at the end of its second quarter, up from about 845 million as of December 31. To be sure, in its IPO filings and other documents, Facebook reports that it considers about 5 percent of “members” to be fraudulent.

The website also claims 500 million mobile users as well as 1.1 trillion “likes” by members since launching the feature in 2009.

On Tuesday, Facebook COO Sheryl Sandberg, 41, announced a new “marketing bible” during appearances as Advertising Week in New York and her own TV appearances.

Analysts expect Facebook to report third-quarter net income of $287.8 million, or 11 cents a share, on revenue of $1.23 billion, according to Thomson Reuters estimates. Facebook reported a second-quarter loss of $157 million, or 7 cents a share, on revenue of $1.18 billion.

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