Platform is essential for non-fiction. Platform means "how do people know who you are NOW" which means it's not "I will build a website" or "I will go on TV" cause those are things you'll do in the future. Platform is about NOW, before the book proposal.
One good way to build platform is to have articles or opinion pieces published in major media. If you don't know how to do that, there's a way to learn. It's called The OpEd Project and they offer seminars on how to pitch yourself and your topic to editors, as well as other things that help you build platform.
I attended the Core Seminar here in NYC yesterday. I think it's well worth the investment to attend. Seminars are held across the country. Their website gives details on the project itself, and the upcoming seminar schedule.