They are the market leader by far, their valuation is high right now, but there are some well funded competitors just coming into the space. Most importantly, their customers have low switching costs.
What's to stop them from losing 30% of their customers to AOL, Yahoo, or Yelp once these new entrants ramp up their operations?
Sell now, while they have the stratospheric valuation and the leverage & profitability to defend that value.
Regardless, that only supports your key point. In terms of the right time to sell, they are peaking and they should.
I think the fact they are out there seemingly shopping their company around says to me: "this b model is not defensible, the valuation is not sustainable, let's offload it."
Facebook Places:
http://www.facebook.com/places/
http://news.ycombinator.com/item?id=1616082
Facebook Deals:
http://www.facebook.com/blog.php?post=446183422130
http://news.ycombinator.com/item?id=1865817
EDIT: Updated style.
Agreed, they have a huge distribution channel but so does for instance Target or Walmart, but groupon just doesnt align to those companies core competencies. Hence not a great fit.