Inc. Magazine:

Solomon [...] estimates that the group-discount market could be worth $1 trillion worldwide.

Okay, Rob Solomon is president and COO of Groupon, so he has an interest in talking that number up, at least in terms of investors (though he may also be encouraging clones), but a trillion is a pretty large number. According to this, the world GDP is "just" $61 trillion. So he's saying the group discount market could account for 1.6% of the global economy (though he doesn't put a timeline on it, so the world GDP may have grown by then to $100 trillion, making the group-discount share just 1%). I don't know enough about macro econ to say whether that's complete bullshit, but it seems like a pretty large number to me. It's a fascinating business, no doubt.