Fundamentally the way in which most people view the feudal system is flawed. As children we are usually shown some variation of a pyramid that clearly and neatly demarcated the ranks of society: King - noble - knight - peasant While that does quickly give you an idea of how the system worked, it leaves out large parts of society. For example, take craftsmen; they formed guilds in most instances and there was a set order within each guild. Apprentices worked in a trade until they became journeyman and then masters. In this instance social mobility was brought about through skill and a fair bit of social maneuvering. There are tons of other example to choose from, but basically people either could claim a noble lineage or they could not. Then they rose or fell according to their ability to gain favor, seize an opportunity, or sheer luck.

It's worth mentioning that despite what my above post would have you believe, there was little social mobility by today's standards. The vast majority of people were peasants who lived and died at virtually the same level of society.