You likely have a fundamental misunderstanding of libel law. With libel, truth is the ultimate defense, and somebody's opinion isn't really libelous as long as it's presented as opinion. Unless the facts in the post are made up, which seems unlikely, the writer is fine.
Groupon smells like a ponzi scheme? Fine. Careful, Groupon could turn out to be a ponzi scheme? Great.
Groupon is straight-up a ponzi scheme? That's pushing it, and would raise any lawyer's eyebrow.
The problem here isn't so much about facts and opinions, or even necessarily the presentation, it's about the term "ponzi scheme" which is very, very legally charged. Check out the episode of Bullshit! on pyramid and ponzi schemes-- not so much for the content of the episode, but for the fact that they avoided saying "ponzi scheme".
Or, imagine this: The New York Times ran this piece, even in the opinion section. Does it seem like a libel case now? Keep in mind that libel and a libel case are entirely different. A company could very well bring a case to court and drain thousands from you in legal fees just to protect their reputation.
And they'd have a pretty good case too. Yeah, malice probably isn't there, but they could certainly argue a reckless disregard for whether or not this was actually true.
