Hi cdogstu99, email and Facebook are two very different marketing channels, so the possibilities for engaging your subscribers are very different, too. Have you tried measuring the return on sending an email newsletter to your subscriber list, vs sales generated by Facebook? The results from this will perhaps best help you determine a) the value of an email subscriber vs. Facebook fan/like, and b) where you should focus your marketing efforts in the future.
Perhaps you should consider promoting your newsletter on your FB page, too. You might also want to add on your page some details as to what's going to be included in the newsletter. For example, do you send your newsletter daily/weekly/monthly? Does it include exclusive, email-only offers, or is it just a round up of new deals? And do you respect your subscribers' privacy? Adding this sort of information will give folks a lot more incentive and confidence to join your list.