I stumbled upon Instafreebie less than a month ago. I had 150 mailing list subscribers back then. Now I have nearly 900, and the numbers are only steadily climbing. The subcribers all came from cross promos a certain facebook group organises, of which I am a part of. The magical thing about Instafreebie is that every time anybody downloads a book, they are automatically included into the authors newsletter.
I recommend Instafreebie to any author, big or small, who wants to reach out to an even wider audience.