Fresh from passing the billion-member mark, Facebook was letting members in the United States pay seven dollars each to have personal posts given priority in feeds seen by their friends. "When you promote a post - whether it's wedding photos, a garage sale, or big news - you bump it higher in news feeds so your friends and subscribers are more likely to notice it," Facebook's Abhishek Doshi said in a blog post. The world's leading social network said the US testing of promoted posts is an expansion of trials rolled out in more than 20 countries since starting in New Zealand in May. "Every day, news feed delivers your posts to your friends," Doshi said. "Sometimes a friend might not notice your post, especially if a lot of their friends have been posting recently and your story isn't near the top of their feed." Twitter has claimed success generating revenue from "promoted tweets" at the globally popular one-to-many text messaging service. Facebook on Thursday celebrated eclipsing the billion-member mark, touting its mission to make the world more social while investors wondered how the service would cash in on its popularity. Facebook shares slid about 1.6 percent to $21.60 in after-market trading on the Nasdaq. The company launched a hotly anticipated public offering in May, but has seen its opening share price of $38 tumble. Analysts are divided on whether Facebook can leverage its massive user base for advertising and other revenues and still remain loyal to Zuckerberg's goal of making it a service to connect the world. Of particular concern was how Facebook planned to make money from members shifting to accessing the service on smartphones or tablet computers. Interviewed on NBC television's "Today" show, Zuckerberg said the company was moving forward despite some rough patches. "We're obviously in a tough cycle now and that doesn't help morale, but at the same time, you know, people here are focused on the things that they're building," he said.