[WS:] This is true, but you also missed an important element of the equation - that the "ruling class" is not a unified group but rather a collection of groups sometimes cooperating and sometimes competing with each other. As Charles Tilly ('From Mobilization to Revolution") argued, some of those elite groups may seek alliances with social movements when they feel that this gives them strategic advantages vis a vis other elite groups. As a result, a social movement MAY - under certain circumstances - pose a significant threat to certain elite/ruling class interests.
The emergence of a social movement that in itself appears weak may alter the balance of power among elite groups. This may encourage an elite group that would normally remain copasetic to actually enlist the support of the movement and make a move against a rival group - which they would not do in the absence of the movement. It is similar to, say, a mid level managers holding a grudge against the big boss but remaining quiet because they feel they cannot effectively oust him , but when they discover that a bunch of employees dug out some dirt on the big boss they may team up to move against him.
In other words, social movements do not face unified elites alone, but rather they alter the existing power balance among different fractions of the elite. They do not need to be strong enough to overpower the elite themselves, all they need to do is to alter the balance to the point that they can leverage more power against a fraction of the elite.
Wojtek