>"Surely the ownership would have to be backdated to around his last involvement with the company, which would be miniscule compared with Facebook's valuation today."

That would be a HUGE amount of money. Even if the subsequent investments are still legit (despite not having the involvement of the main investor), Zuckerberg owns like 30% of Facebook, so if you assume the primary investor wouldn't have gotten diluted worse than Zuckerberg, you're looking at tens of billions. Even if he gets diluted as bad as Saverin did (down to 1/7th), that's still like $5 billion or so, right? That's shit-tons of money.