You can delete stuff or pages, and you can delete your account altogether, although people seem to have problems with the latter, mainly because Facebook deactivate the account in question for two weeks, and permanently delete it ONLY if you do not use Facebook IN ANY WAY during that period. So checking to see if it's gone will count as use, as will any action on other Facebook pages, or logging into other services using Facebook Connect .. which imo is a bit sneaky!
As to its being erased or simply hidden from view .. on the internet I'm not sure if anything is permanently and completely erased, or still floating in the ether if you know how to find it. Someone more versed in the internet may know.
As to it's "value" .. its real value will only be tested if it's sold or floated; but I guess the recent value put on it is based on its potential (as are most valuations of companies) plus it's an implied value really, based on its phenomenal growth and the recent investments of some $500m in it.
Its potential for advertisers is huge because so many people put so much personal information on it, including their interests, likes, activities etc, Facebook can analyse that and target advertising, which makes it more of a winner for advertisers than almost any other site selling ads; a massive selling point. That alone must vastly increase its potential value. (And is why the ads appearing on mine are so random!

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Caryl