1. Meebo Messenger was great, but was not pulling in nearly enough revenue
2. Meebo tried to pivot to an advertising bar for more revenue with no luck
Understand that Google probably didn't buy Meebo with the intent of shutting its services. Meebo was most likely already failing (hence the 70MM overall raise and 100MM exit), and would have had to shut down anyway. However the founders were, luckily, able to exit the company first.
Meebo was great at its peak, but Google didn't kill Meebo. It was already dead.