Actually it's been widely discussed among us fund-raisers. He raised his money in some very different ways, including social media. That, coupled with the appeal to a younger demographic who used that social media and the willingness and ability to accept international donations online really made a difference. His team also employed much better "donor relations" practices.
A well known researched gave equal gifts to each of the following campaigns during the primary, and tracked how the gifts were thanked:
McCain: thank you letter three weeks later.
Clinton: thank you letter two weeks later.
Romney: thank you letter two weeks later.
Obama: thank you text or email within minutes, followed by additional information and contacts from various members of the team. A mass text was sent out to all donors when Obama was ABOUT to accept the nomination, thanking the donors for making it possible, and asking them to make another gift. This created a sense of partnership with his donors, which encouraged additional gifts from existing donors. When they had a bad day... they texted "we had a bad day... help." When they had a good day, they texted "we had a great day, help." And then they thanked some more.
The fact is that Obama raised all that money because his team raised money in a very sophisticated way, compared to the rest. It's a place where youth helped.