we know the leaders in first two and what they went through before becoming what they are now. (AMZN was $ 2 stock and google was a also ran to yahoo ) . the leaders(FB &ZNGA) of next 2 category are breaking away from also ran in there respective categories . FB and ZYNGA are going through the same scrutiny what AMZN and GOOGLE went through in the initial years. But since they have invented the respective share and play categories it will be very difficult to for any upstart to catch up with them.