How the quickly the media world turns on it's axis

Like many people I saw the 'Facebook film' Social Network the other day; Jesse Eisenberg is completely believable as Mark Zuckerberg and the Aaron Sorkin script is brilliantly balanced across a maze of intriguing characters. The film signs off by reminding us that Facebook is worth $25 Billion today. Really? And exactly what credible logic went into that valuation?
The (probable) argument runs that with 500 million active users worldwide that the user base is too 'locked in' to Facebook to move elsewhere....I mean you wouldn't, would you, all of your friends are already on Facebook so why move to a new or rival offer? Facebook has the momentum and whilst the contextural targeting revenue doesn't currently come close to justifying the $25 Billion valuation (unless you want to apply la-la land multiples) it's only a matter of time.
That sounds awfully like the kind of investment banker flaky nonsense peddled in the first dot.com boom...and where did that go? Cash, not eyeballs or headcount, is king. Meanwhile News Corp has just announced a 3.2% increase in revenues (3 mths to end Sept 2010) with growth from 'old media' and the notable low light of a 25.5% decline in revenues within the Digital Media Group division (including My Space). Within this Rupert Murdoch is trying to change the free content model. All of a sudden digital is looking like a potential balance sheet drag.
It's not, and never will be, given the way it's changed the (profitable) retail landscape and consumer behaviour but within that there are no sacred cows including Facebook. What marks out so many digital business models (including Facebook) are the low barriers to entry and fragility of brand loyalty...you're one mistake away from oblivion. Rather than the 500 million active users being the fortress walls that repel new pretenders and the possible renaissance of the old guard (like My Space), it could be the achilles heel and a reminder of that digital brand fragility. Given any abuse of the customer relationship with Facebook (data confidentiality?) the reaction may not be individuals leaving one by one but entire friendship communities up rooting and moving elsewhere...and of course that binnary, 6 degrees of seperation, benefit that once sent Facebook into the stratosphere suddenly consigns it to the dustbin of yesterday's 'not cool' stuff. $25 Billion...really? I'm not so sure.