However the problem with your math (and most others in this thread) is this: lets say Zynga values me at 4 shares, and I get 1 vested per year. However after two years, and a few splits, I have 128 shares.
The company hired me to work as a 4 share employee, then it grew like crazy and I became a 128 share employee. This is normal in tech nowadays, most people take it for granted that you just get lucky and accept your windfall. But the fact is that you are a 4 share employee being paid 3000% of what you were hired at.
You're not a "4 shares employee" you're a "% of the company employee". The 4 shares are worth some percentage of the company and after all those splits they're still worth that same percentage. Splits usually happen to get the cost of the overalls hares down but a $1m holding in some stock is still worth $1m after the split.
>But the fact is that you are a 4 share employee being paid 3000% of what you were hired at.
So what! This is the point of accepting stock as compensation. They worked below what they were worth for the hope that they would win the lottery. They did win the lottery and now the company wants a redo.
Zynga shouldn't be able to come back, with 20/20 hindsight, and say "turns out we did better than we thought we would so we're taking your stock back".
If they failed they certainly wouldn't be giving employees other compensation to make up for their stock being worthless.
IANAL, but if the termination is motivated by the success of the equity gamble the employee took up front, that looks more like breach of contract. Don't want to keep them? That's fine, but pay what you promised you'd pay.