In my experience, very few, but most of those were exceptional people. They contributed to something they were passionate about, and learned all the stuff you don't get in school (writing code that can be maintained for a while, and that's maintainable by others; use of source control and issue trackers, etc).

That said, I can think of two cases where I'm pretty sure they "contributed" because they were told or read that it would be good to do for their resume. One basically just uploaded a couple projects to github (with one commit). The other had contributed a few trivial patches to a couple projects, at least one had some comments that were unaddressed and didn't get merged.