Is it a cash deal though? If not, it might take a few years until their stock vests and until than zynga might not be worth much. So i really hope it's at least 50% cash.
Zynga could go bust. Without being acquired OMGPOP could go bust. So the investor would have to weigh the risk.
I also think there would have to be some cash and/or stock that would be available right away, for it to be an enticing deal.