I had a gathering at my dorm in 2008 for those like me who refused to vote McCain or Obama.
Filled with NPAs, demmys and elephants, there were dozens of 20 somethings in a single floor of the dorm; they weren't alone. 10 here, 100 there, it adds up.
It's not a lot, but there are a sizable number of people, specially disillusioned youth, that choose not to vote, regardless of party; I speak from experience.
In some counties, people like this CAN make a difference and change the outcome by not voting, usually, letting the more popular in the district more easily win, but making a difference nonetheless.