Why Facebook has a $300+ billion market cap ... it's all about cashing in on the precious information that "users have chosen to share" (bold and bold italics mine):

"We generate a substantial majority of our revenue from advertising ... We allow advertisers to select relevant and appropriate audiences for their ads ... We believe that a more valuable advertiser and user experience is created through our ability to match the most relevant ads to each of our users based on his or her connections, demographics, and expressed interests ... Advertisers can specify that we show their ads to a subset of our users based on demographic factors such as age, location, gender, education, work history, and specific interests that they have chosen to share with us on Facebook or by using the Like button around the web or on mobile devices ... Performance-based online advertising has typically involved advertisers seeking a specific user behavior such as a click on a search ad or a keyword-based content ad, a response to an email campaign, or an online purchase. We enable new forms of performance-based advertising, where advertisers can connect with users who are likely to have demand for their products based on the information that our users have chosen to share. We believe that performance-based campaigns on Facebook allow advertisers to offer their products to users with inferred intent and enhance users’ experiences by showing them relevant ads tailored to their specific interests ... We also have a global sales force that is focused on attracting and retaining advertisers and providing support to them throughout the stages of the advertising campaign cycle from pre-purchase decision making to real-time optimizations to post-campaign analytics."

(From their Prospectus)