Meh...when you think of the pinnacle of established web stocks the obvious answer is Googles so lets use their valuation as a benchmark. It's hard to compare FB to other companies because its the trailblazer for new social media but nonetheless I think that FB (which is also OVERvalued) is striving to be google.
(note: this is without even factoring the patents and intangibles of google lol)
Sales: 60 Billion a year
Market cap: 400 Billion
Multiple of 6.5. (a lot of this multiple is the patents and intanglibles)
My second choice would probably be Ebay (i'd say Amazon but they had a bad year so their metrics are off)
Sales of 17B
Market cap of probably 70B
Multiple of about 4.5
Yahoo has a multiple of around 7.5. Priceline's is 11 (but Expedia's is only 2.5

).
Lets just use an average multiple of 7 for established web companies.
What's app would have to eventually generate or contribute to making $2.5 Billion in revenue a year for this acquisition to make sense in the long run. I dunno if thats possible. FB is a relatively volatile stock and its cost of equity is presumably high making it that much harder to make a good ROI especially if they keep diluting the shares with these acquisitions. I havent gone deep at all but if you seriously crunch the numbers it has to be overpriced. And I aint even comparing to mass media cause then the metrics would be completely absured. Wall street is gaming these tech companies. Like Poitier said its a bubble I think.
FYI: FB's P/Sales ratio is 20

which is fair for now. But the expectations of these new age tech stocks seems too optimistic for me. There's probably a lot more that we cant see though with respect to how they will use it.