International Fundraising eConference Roundup: Digital Advocacy Tips From the Obama Campaign

As for social networks, he said users get out of them what they put into them. He recommended nonprofits select maybe two social-networking sites to invest their energies in, make sure they have the staff to handle the information flow, and give people what they want.

Both channels were effective for the campaign, but are different in some very key ways:
E-mail: You own the database.
Social networks: Supporters are not in a database. You might be having conversations with people interested in your organization, but they might not want to give you money immediately.