When Facebook went public last week, the company was valued at $104 billion, an astonishing figure for an Internet company. Is the figure preposterously high? That depends on how you look at it. The value in Facebook lies in its enormous audience—901 million people every month who are potential viewers of advertisements and buyers of virtual goods. So you could think of Facebook this way: it is worth about $116 for every user it has. On that basis, the company isn't being valued as highly as Google ($200 per user) or even the question-and-answer service Quora ($145). This metric also reveals that investors are much less optimistic that Twitter (worth less than $60 per user) or Tumblr (about $8) will profit handsomely from their very large audiences.