I don't think they are undervaluing it by saying $19 billion is too much. Mobile users and developing market users should be valued less, especially since mobile ads are less profitable, and developing market users are monetized at a lower rate.
I think they are taking a rising competitor out of the market, especially a competitor that was being pursued by a major competitor (Google). That's the only way you go from a $10 billion bid to a $19 billion purchase price.
And I personally think social/IM clients are still inherently risky propositions, given their history of rising and falling in line with innovations and fads. The phone number feature is innovative, but is already mimicked by other clients.
If you mean the specific value of $19 billion, I agree that they ended up overpaying as a result of the bidding war. By "undervalued", I'm talking about people who are shocked that this company's value is to the order of billions, which I think is reasonable for a user base of that scale in the hands of a company like Facebook.