It's almost as if people derive personal satisfaction from poking holes in Groupon's success (or any startup, for that matter). Groupon has no viable business model, people will get tired of Zynga's games, Twitter can't monetize, Facebook will be killed by Google+/is useless/the ads on it aren't worth anything, etc. Every time one of these things gets pointed out, people show up and start high-fiving each other, say…
I think Twitter is in a far better position...it has taken me a while to realize this, but the truth is that they don't need a business model, really, they can just sell their data to people who do have a business model (like Google). Twitter will sell when it can get a really good bidding war between FB and Google, and maybe MS too. Now, they may screw it up and only get a few million, but this is still going to be a huge success by non-silicon valley standards.