The organised crusades (2nd, 3rd, etc.) were largely professional affairs with knights and men-at-arms making up a large proportion of the forces. Feudal levies could only be raised for limited periods of time, after which they had to be paid. For a long duration event like a crusade, the forces were made up of various professional and semi-professional forces.
However, there was a constant flow of crusaders from a range of backgrounds, including peasants during the crusades as well as some 'spontaneous' peoples crusades.
The 1st crusade was also a more complex thing, with some limited evidence that a significant proportion of the forces involved being various lower-class religiously motivated people. The chroniclers were only interested in the deeds of the nobility, but there are a few references that are suggestive of large numbers of non-noble crusaders outside of the formal feudal structures of the crusade.