I did not see a forum specific to e-mail marketing/list building, I apologize if I overlooked it. The money is in the list I understand this but viral marketing is extremely powerful as well. Is anyone using a method to have a new subscriber and have them join a fanpage in one single action or this a gradual thing? I suppose you could possibly offer something else on the thank you page? I'm sure there is a product out there to have a user like your page to gain access to a free product but then also snags their e-mail address but that would also be pretty shady since they are not opting in.
Generally, as long as you are not overdoing it you can do some cross promoting. If you have something relevant to something on your social media, you can so go check it out on our page and while you are there like us. And every once in a while throw an opt-in up on your social media pages to get them on your list because inevitably, there will be people who are on one, but not the other.
http://corp.wishpond.com/how-to-run-a-facebook-contest/ Not sure if you would find this helpful or not...but I thought I'd put it up here anyway. You should be able to get some emails using this method. I don't think it would be terribly difficult if the incentive you're offering is worth it in the eyes of the one giving their email address. With all of this, however, remember that even an email address is not an actual conversion until it becomes a sale. That's an even longer road! Good luck!
Are you looking for emails or likes or both? There are social locker plugins for WordPress that can help you create a setup that requires your visitors to like your Facebook page before they can download any product you are giving them for free. List building is one of the best IM strategies and is way more profitable so I suggest you do not make the switch. Instead, focus on both methods (email and viral). Have you read viperchill.com? It's a popular and excellent blog on viral marketing.