Facebook has officially passed the one billion-user mark — a remarkable achievement for a company that’s only eight years old, but also a challenge to its quest for sustained growth.The social media giant, which has endured a bruising four months in the stock market since its May 18 initial public offering, hit the milestone in September and has since acknowledged that a slowdown in new-user acquisition is inevitable as its worldwide reach expands.But while Facebook shares trade at 45 times projected 2012 earnings, doubts over whether the social media giant can squeeze more and more money out of its members have shaved more than 40 percent off the company’s value since its IPO.The company said late last week it has seen 1.13 trillion "likes," or endorsements by users, since it launched the feature in February 2009.It also said it now has 600 million mobile users, up from 543 million at the end of June."There are five billion people in the world who have phones, so we should be able to serve many more people and grow the user base there," Facebook CEO Mark Zuckerberg said in a recent interview with NBC.While the company continues to try and expand its user base, it’s also exploring ways of making money beyond advertising, which accounted for 84 percent of the total revenue in the second quarter.Promoted postsFacebook has long declared that it's "free and always will be." And it still is — unless you want more friends to see what you have to say.Late last week, the company launched a new feature in the US that lets users pay to promote their posts to friends, just like advertisers do. Facebook has been testing the service in New Zealand, where is tries out a lot of its new features, and has gradually introduced it in more than 20 other countries.Facebook says that promoting a post — such as announcing a garage sale, charity event, or big news like an engagement — will bump it higher in your friends’ news feeds.“Every day, news feed delivers your posts to your friends. Sometimes a particular 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,” wrote Abhishek Doshi, a software engineer at Facebook on the company’s news site.Facebook hasn’t said exactly how much it would cost to promote posts, only that it’s considering a range of prices as part of the test. Those who currently have access to the feature say a promoted post typically costs around $7.“Want” buttonFirst there was the “like” button but soon, there may be a “want” button. Facebook is now testing a feature that lets users create “wishlists” of home furnishings, clothing and other retail products, laying the groundwork for what some believe could be another push into e-commerce.Facebook said it is working with seven retailers, including Pottery Barn and Victoria’s Secret, to test the new feature that will allow certain users to flag images of desired products by clicking the “want” button.“People will be able to engage with these collections and share things they are interested in with their friends. People can click through and buy these items off of Facebook,” the company said in a statement.Those familiar with Pinterst and another, lesser-known app called “Want!”, which is like an Instagram for window shopping, will see many similarities.The feature, which has been dubbed Collections, could help Facebook play a bigger role in the online commerce market by encouraging its one billion users to buy products for their friends and by sending shoppers directly to online stores.A Facebook spokeswoman said the company does not receive a fee when someone purchases a wishlist item on Facebook from a retailer’s site.The company is testing the Collections feature with Neiman Marcus, Michael Kors, Smith Optics, Wayfair and Fab.com.Some users will see the “want” button as part of the test, while others will see a button inviting them to “collect” an item or to “like” an item.Unlike Facebook’s existing “like” button, the feature that Facebook is testing will showcase the “liked” item within a user’s Timeline profile page.Tech analyst, Colin Sebastian of Robert W Baird, said the new Collections feature could open new sources of revenue for Facebook.“E-commerce is one of the best ways to monetise the internet,” he said. “Thinking about how large they are as a platform and how engaged people are, there are lots of levers they haven’t pulled yet in terms of monetisation.”In addition to potentially collecting a transaction fee for referring users to an e-commerce site, Mr Sebastian said that retailers might also pay Facebook to promote products featured on users’ wishlists, similar to the way Facebook’s current ads function.The promoted posts and “want” button features come just one week after Facebook unveiled a feature that lets US users buy and send gifts such as coffee, pastries and gift cards to friends — signalling the company’s intent to play a bigger role in e-commerce.