The 2013 ECAR Study of Undergraduate Students and Information Technology, which polled more than 113,000 US and international undergraduates, includes some interesting numbers regarding student use of free and/or open educational resources.  From pages 11-13 of the report:

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Freely available course content/open educational resources, e-books, simulations and education games, and e-portfolios are still in the experimental stages for most students. Seven in 10 students (71%) say they have used freely available course content/open educational resources (OERs) in the past year, yet for most students the scale of use is nominal. Only about 1 out of 10 of these students use OERs “all the time” (Figure 5).

Regional and Carnegie class differences were not noteworthy, but older students (14%) compared with younger students (9%) more frequently report that they use OERs “all the time.” In looking at responses to an open-ended question about how students recommend that their instructors use freely available course content, we found that most identified, at least vaguely, ways that they imagined bringing free course content into their studies: as learning aids, as supplemental information sources, and as providers of different perspectives on topics.

They cited the value of sourcing additional examples and revisiting/repetition of complex or key points outside the confines of class. The majority of respondents identified a resource or activity related in some way to their academic goals. Khan Academy commonly surfaced as a supplemental OER that students employ independently or, less commonly, as prompted by their instructors.

One student’s comment exemplifies the supplemental value of this alternative: “Sometimes taking notes and listening to a lecture [by] the same person can be like bashing yourself over the head with a textbook if it doesn’t make sense. But it’s nice to listen to other styles of teaching like Khan Academy. It gives a valuable perspective.”
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As has been pointed out in the OER community, the report largely misses the distinction between free resources and openly licensed resources, which is important.  Nonetheless, this is a clear indicator that most students, at least occasionally, look beyond the materials presented to them for additional explanations of key concepts and for supplemental materials.

Ten percent using “all the time” and 30% using OER “on occasion” may sound modest, but there were roughly 18 million undergraduate students in the US in 2007, so this indicates that there are more than 7 million US undergraduates using OER and free learning resources and millions more aware of them and trying them out.