I just don't believe "what they can get" should be one of our options.
Ideally, there would be an impartial entity responsible for settling these valuation disputes (how do we handle this between corporations?). Heck, I'd even take using market valuations (which isn't the same thing as whatever the IRS can get from FB).
If I report a transaction for a value considerably lower than the estimate they'll investigate it which usually would result in a new estimation and if that one is as well differs considerably than the transaction amount there would be an investigation regarding possible tax evasion.
Corporations aren't different when they exchange assets the valuation figures are reported to the authorities if the regulatory body thinks the estimates were set higher or lower than their actual value they can choose to open an investigation and if need be fine or prosecute the offending parties.