I leave it to New York lawyers to get into the fine points of the contract law here but I would add this to my earlier comments:
1. This contract is outrageous and unconscionable - not one founder among the thousands I have worked with over 25 years in Silicon Valley would even begin to consider giving away half or more of his company (much less 84%) for a $1K investment. If I tried to do this as a lawyer in a client's company, I would be instantly disbarred. This type of money typically gets someone a 1% interest or less, even at the earliest stage.
2. When I pay x dollars for item y, and the other party breaches the contract by failing to deliver y after I have paid my x dollars to him, I have been damaged and am entitled to a remedy. The normal remedy is an award of damages, which means I am normally entitled to the monetary value of y as of the date of breach or, if I rescind the contract for the failure to perform, to a refund of my x dollars. Applying this principle in Mr. Ceglia's case (and assuming he gets the best possible outcome in establishing liability), he would get either the monetary value of 84% of what Facebook was worth in the summer of 2004 (when it was first formed and when he became entitled to his equity interest) or else to a refund of his $1,000 - in each case, with interest accruing since the date of breach. If we assume Facebook is like the typical startup, and was worth at most a few hundred thousands of dollars at inception, Mr. Ceglia's maximum damages would be up to a couple of hundred thousands (plus interest since 2004).
3. For Mr. Ceglia to be get a judgment ordering Mr. Zuckerberg to convey 84% of his stock in Facebook to him, he would need to show that he is entitled to specific performance of the contract. This is an equitable remedy and a court will award it only where the item that was to have been a delivered has a "unique" aspect to it that cannot be compensated by damages (the classic example is a parcel of real property). I would doubt that stock would qualify as "unique" in this respect. Also, because specific performance is an equitable remedy, a court will only use such a remedy to do equity and will therefore not grant it where (just to pick a few items off the list in a Wikipedia piece on this remedy, http://en.wikipedia.org/wiki/Specific_performance) (a) it would cause severe hardship to the defendant, (b) the contract is unconscionable or illegal, (c) the claimant misbehaved (no clean hands), (d) specific performance is impossible, or (e) the contract is too vague to be enforced. As appears from that list (assuming that New York law generally conforms with this approach), Mr. Ceglia has some daunting obstacles to overcome before he can claim a share of Mr. Zuckerberg's holdings, even if he can prove that he was harmed by a breach of contract. Of course, those same equitable factors utterly preclude his being able to get specific performance in any way that would harm Facebook's innocent shareholders generally.