Our point of view is that FB should be valued based on revenue potential from total minutes spent on FB times its powerful margin expansion engine." Martin contends that monthly users are the best proxy for consumer acceptance, "since consumer substitution is easy online." She notes that the company's 900 million users after 8 years in existence compares to the roughly 1 billion Google users after 14 years. She also notes that Facebook now accounts for 14% of time spent online globally, suggesting revenue potential of $14 billion globally and $6 billion in the U.S., based on estimates of $98 billion in global Internet revenue in 2013 and $46 billion in U.S. Internet revenue. Martin also contends that operating margins at Facebook are both enormous and expanding: 33.7% in 2009, 52.3% in 2010, and 47.3% in 2011. "This powerful economic engine suggests profit growth will be faster than revenue growth," she writes. The biggest execution risk for the company, she notes, is monetization.