I find it interesting that a Groupon user ends up being so highly valued by the market. I wonder if it's because Groupon can make money from each user in ways other than advertising.
I doubt it - Skype is last on the list yet first on the list of paid products. The fact that groupon's rev comes from a percentage of sales doesn't really mean they aren't advertising based, anyway. It's pretty much the same as running CPA (cost per action) ads.
I think common wisdom is that they're valued based on the size of the local ad market and because of how hard it's been for others to break into local.
My understanding is that Skype makes the bulk of their money off of international calls bridged Skype to that country's local telephone service (which is often substantially less than calling cards, or direct long distance).
