25% of Groupons purchased in December were bought on a mobile device

"Groupon today announced record mobile performance in North America in December 2011 with one quarter of all Groupon vouchers purchased via a mobile device and both consumer and merchant app usage up significantly year over year.
December 2011 North American Mobile Performance:
25% of Groupons were purchased on a mobile device
Mobile app users increased by more than three times from last December to over nine million
More than 12,000 merchant partners used Groupon's merchant mobile app to redeem vouchers and track purchases using an iOS device or an Android phone."

Source:  Press release from Groupon, 27th February 2012