WHO-HD in Des Moines began the month of April with 13,182 likes on Facebook. That’s a respectable number, but it wouldn’t even put the station in the top half of the list of top TV stations on Facebook.

Twenty days later, the NBC affiliate is about to pass the 30,000 fan threshold (29,536 as I type this). The graph below shows the station leave the local newspaper in its dust and gain considerably on another local TV station, KCCI, leading into the May sweeps.

How did WHO do it?

The station gave away a prize each of those 20 days, including four iPad 2s. And it promoted those prizes every half hour on air.

“When we ran that (promo) during The Biggest Loser, they came flooding in,” said Tim Gardner, director of creative services for the station. “Definitely on-air drove this.”

WHO has run 20-day contests before, requiring viewers to see a special code on a newscast and enter that code online for a chance to win.

This was the first time the station ran a contest entirely through Facebook and didn’t require a special code (it’s part of a corporate initiative to grow Facebook fans across all stations, so expect to see this happening in many more markets).

Facebook has some fairly strict rules for contests, and WHO used Woobox to make sure everything was within the rules. Woobox builds in incentives for users to share a link to the contest with friends, which allows the viral nature of Facebook to take over.

A few takeaways I got talking to Gardner:

• The better/cooler the prize, the more entries you will receive. iPads were getting about 50% more entries than $250 gas cards.
• WHO’s goal was to double fans, and that goal was reached 13 days into the contest.
• The contest was a little slow to take off (in terms of entries and new likes) and it slowed down toward the end (in terms of new fans, not contest entries), so perhaps a similar effect could have been achieved in fewer than 20 days. Here’s a breakdown of how many new likes WHO gained each day, according to Wildfire Monitor:
• April 1-2: 234
  April 2-3: 253
  April 3-4: 344
  April 4-5: 537
  April 5-6: 3,600
  April 6-7: 1,559
  April 7-8: 890
  April 8-9: 2,161
  April 9-10: 432
  April 10-11: 366
  April 11-12: 1,599
  April 12-13: 844
  April 13-14: 739
  April 14-15: 474
  April 15-16: 310
  April 16-17: 191
  April 17-18: 146
  April 18-19: 444
  April 19-20: 906 (the final iPad giveaway)

The challenge now is to take advantage of the growing following on Facebook. Gardner said the station will be much more attentive to Facebook, with more staffers now serving as admins on the station’s Facebook page.

What’s all this worth?

By my calculations, WHO spend less than $6,000 on prizes, plus $250 for Woobox. So let’s say the station spent $6,250, plus whatever man hours went into this project.

Using this article that estimates the average Facebook fan value at $136.38, WHO gets a return of $2.2 million for that $6,250 investment. Not bad for 20 days.