Starbucks’ 6.5 million Facebook fans are worth the equivalent of a $23.4 million annual media spend, according to calculations by social media specialists Vitrue, reported in Adweek.

The firm has worked out  that, on average, a fan base of 1 million translates to at least $3.6 million in equivalent media over a year, or $3.6 per fan.

Vitrue arrived at its $3.6 million figure by working off a $5 CPM, meaning a brand’s 1 million fans generate about $300,000 in media value each month.

Vitrue analysed Facebook data from its clients’ 41 million fans and found that most fans yielded an extra impression. That means a marketer posting twice a day can expect about 60 million impressions per month through the news feed.

The figures don’t take into account the most powerful incentive for brands building fan bases: social customer-relationship management. Marketers often use their Facebook hubs to inform fans of new products, services and promotions.

“When you start to [add] engagement value, it goes higher,” said Michael Strutton, chief product officer at Vitrue, “we’re trying to get an easy-to-understand valuation terminology.”

Increased website traffic remains the main metric marketers use to measure the success of social media campaigns, according to a report earlier this week by eMarketer, underlining that social media measurement remains a fairly blunt instrument.

Only half of those marketers in a Datran Media study believe social media will generate quantifiable results in 2010, with 37% unsure and 12% saying no.