Platforms

I got a very lovely rejection (yes, Virginia, there is such a thing as a nice rejection) from an agent some time ago. Among the reasons that she didn't take on the book was that I didn't have a platform. I'm an old school kind of girl, born in the days before cell phones and i-pods and blogs. I thought a platform was something you waited for the train on. Or maybe the kind of shoes we wore in those days before cell phones and i-pods.
I'm not sure how one builds a platform, exactly. I know that it doesn't involve heavy machinery. And that it should have something to do with something that you know something about. If you write a book about the shorebirds of North America, for example, you could do a blog or a website about shorebirds or about wild birds in general.
It's more problematic for fiction writers and poets, though. As a fiction writer, I'm not an expert on much of anything. I might do a bit of research here and there. For one story, I did some research about pediatric cardiology and for another, research about ocean life. But this hardly qualifies me as either a pediatric cardiologist or as a marine biologist. I'm like the commercial that says 'I'm not a real doctor, but I play one on TV."
So the question becomes, how do you do this platform thing? The blog here is step one for me. I do know something about something about writing. And, if any one's interested, I can share.