As much as I would like to believe that the reason Obama won the youth vote by 26% was because so many millions of 18-34 yr old would-be Ron Paul supporters either voted third party or stayed home, the numbers don't bear that out.
Ron Paul got two million votes. 18-30 yr olds made up about half of that so, we'll say one million. There were about 115 million people that voted for either Obama or Romney. Even if only 20% of them were 18-30, we're talking about 14 million 18-30 year-olds that voted for Obama. Assuming that 80% of Ron Paul's young, primary supporters did not vote for Romney (I'm 21 and I did, so, I would fall into the hypothetical 20%) and Dr. Paul managed to double that amount with young general election win-overs, that gives him about 1.6 million more young votes than Romney got. In other words, Obama would have still won the young vote by a comfortable margin even running against Ron Paul. Only instead of being 63%-37%, it would have been more in the area of Obama 55% Paul 45%.