The civil service examination system is not equivalent to institutionalized meritocracy. It was an institution created in an attempt to solve the problem of preserving meritocratic governance over successive generations, and despite being the best solution conceived to date, ultimately failed. The system may have worked for the first half of a dynasty's lifespan, but would fall apart due to corruption, bribery, and the purchasing of offices.
In theory, China during and after the Tang dynasty did not have a class of nobility or aristocracy, but instead a class of scholar officials arose. In practice, government offices were monopolized by the scholar official class, as only this class had the resources and wealth to finance the long and lengthy education required to pass the civil service examinations. There was no more social mobility than in Greece or Rome - you were either born into a family with means or you were not. Even a basic education to attain rudimentary level of literacy was not within the means for over 95% of the population. Despite the imperial examination system, high government offices and wealth would, over the course of the dynasty, be concentrated in the hands of only a few dozen families.
Ultimately, the dynasties that adopted the civil examination system (Tang - Ming) did not last any longer than the Han. At the beginning of the dynasty, when wealth/land had been redistributed and power centralized, you had a relatively large degree of social mobility and meritocratic rule. However, over the course of generations, wealth and power would be monopolized by a small number of families, social mobility becomes impossible, the livelihoods of the common people suffers, and the state becomes weak and ineffectual.