Only 20% of Groupon subscribers have ever made a purchase

"Here’s the deal:
Groupon had 115.7 million subscribers at the end of June.
Of those subscribers, only about 20 percent — or 23 million — have ever made a purchase. Those are called cumulative customers.
The average subscriber (not customer) spent $18 in the first half of the year, down from $21 last year.
The average customer over the lifetime of a membership has purchased four Groupons, up from three a year ago.
The average revenue per Groupon sold in the first half of the year was $25, up from $23 in the same period 2010.
The number of merchants Groupon worked with in the first half of 2011 increased to 135,247, up from only 12,468 in the first half of 2010.
Converting subscribers into customers will be key for Groupon in order to justify its high marketing costs."

Source:  Figures revealed by Groupon as part of it's Q2 2011 results, reported by AllThingsD, 10th August 2011