Facebook has a real business model and lot of additional revenue they've been leaving on the table since they didn't have shareholder pressure. Groupon is a glorified email list/call center with a very low barrier to entry. Huge difference.
Of course, that doesn't mean Facebook isn't going to be overvalued out of the gate though...
I would really like to see them take on adcents. Ya, i do agree facebook is a bit more of a power house then Groupon. (to say the least)