Social networking sites can help you either in promoting your business through email list. You should contact the best people or establishment that offers the type that you want.
If you want to build a list of subscribers, you have to give something away for free, it could be an e-book, a video about something, an invitation to an event somewhere, etc. There are many things you can give for free to your audience in order to build your list. I hope that helps a little bit.