Now days social networks allow you to grow your mailing list. Subscribers have become more susceptible to the information they want to receive in their inbox. For those two reasons you must take advantage of the tools provide it by social networks; to show readers how good your newsletters are and encourage them to subscribe to receive them.
So it is best that aside from the subscription form you have on your website, you should also include it in your Facebook page. For example, you may have some users that visit your Facebook page and would never enter your web page, so if you have your subscription form available in your fan page your customers will be able to subscribe to receive your newsletters. So putting the format of the subscription form in your Facebook page will not only help you grow your list but it will also keep your list ICANN compliance since all emails will be opt in.