As a writer and consultant, part of what I do professionally is advise authors how to use the Internet to build their platforms. In publishing parlance, “platform” means the total number of people you can reach about your book. It’s the sum total of readers, TV viewers, podcast listeners, email and blog subscribers, Twitter followers, Facebook friends, attendees at public speaking events and readings, stalkers, and so forth.
The theory is, the bigger your platform, the more buyers you’ll have. Also, your friends will have friends, and they’ll have friends, and so forth, kicking off a word-of-mouth effect and reinforcing your relative influence (because the more people who appear to be talking about you, the more popular and important you must be).