Facebook now has 750 million users and reaches 43% of all Internet users worldwide, according to Alexa, up from 35% a year ago.

Facebook now has 750 million users and reaches 43% of all Internet users worldwide, according to Alexa, up from 35% a year ago. Facebook now hosts perhaps 4 million fan pages, branded sites operated by companies, celebrities and enthusiasts.

One of them is the President of the United States. Barack Obama's campaign website was attracting 8 million unique visitors per month as the 2008 election approached. This year Obama announced his reelection campaign on Facebook and has integrated Facebook's registration system into barackobama.com. "We may never see another [freestanding] campaign website that gets that kind of traffic," says Mindy Finn, a partner at social media advisory EngageDC. "All of that activity has just moved on to other platforms."

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The most common interaction between consumers and brands on Facebook is to either "like" a brand as a fan or "share" some piece of that brand's content such as a video or photo. The meaning of these gestures is ambiguous. Some users "like" a page in order to announce their affinity to friends; others respond to an incentive such as a free sample in exchange for publicly associating themselves with a brand. Once a user has "liked" a page, updates from that page show up in her Facebook news stream where her friends can see it, too--an important hook for viral marketing.

Lots of marketing types have tried to peg the value of a Facebook fan. Their methods and estimates vary widely, reflecting the new-and-untested nature of Facebook marketing: Group-sales service ChompOn found that the average Facebook "share" led to $14 in sales at the company's website. Vitrue, a consultancy, figures that bringing a customer on as a fan is worth between 44 cents and $3.60 in increased sales from the engagement that Facebook encourages.

Facebook has suggested that people who want to use Google Analytics or Omniture should use links on their Facebook pages to send visitors to sites where those services are installed and then analyze the traffic that results--sort of like watching shadows on a wall to figure out what's going on in the next room.