Increasing smartphone adoption is driving sales at daily-deal provider Groupon Inc. 25% of all Groupon voucher purchases in North America in December were made with a mobile device, Groupon reports. Groupon declined to disclose the value of the mobile purchases, but in 2011 it had overall gross billings of $4 billion.

And more than nine million consumers used a Groupon mobile app in December, a three-fold increase from December 2010, Groupon says. Groupon would not provide data for November. Groupon’s app is available for Apple Inc.’s iOS, Google Inc.’s Android, BlackBerry and Windows Phone devices. Groupon also announced it is developing an app for Nokia devices, but would not say when it would be available.

Groupon’s mobile commerce performance does not surprise analyst Nikki Baird, managing partner at research and advisory firm Retail Systems Research LLC. There’s one simple reason why Groupon and mobile go together, she says. “In order to redeem a Groupon, you have to have it in hand, or you have to be able to show that you have it via their mobile app,” Baird says. “That’s a great way to train people to look primarily to the mobile app because it’s way more convenient than remembering a piece of paper.” Many consumers print a Groupon and bring it with them to the merchant for redemption.

“Once you get over that initial hump of mobile adoption, then it becomes easier to get people to check out the app for purchases on a more regular basis,” Baird notes.