Immutability doesn't actually solve much in the way of real-world problems. Most contract lawsuits don't concern accusations of secretly forging altered contracts, or any disparity between copies: they concern interpretations. And if you've ever read a typical boilerplate contract for most any serious transaction, you'll see they cover "if-then" situations about as much as can be reasonably be done.

Explain the mechanism by which a smart contract in this instance would both have made it impossible for Facebook to make this move and, if they did anyway, avoided the necessity of Palmer to sue them over it.