One simple mistake: Do-not-reply@CompanyWhoWillNotAnswer.com
First things first, if you email customers you need to dump the "Do not reply" addresses. Also, you must make it easy for customers to contact you, and you need to reply to them.
When a company has an active Twitter account or Facebook fan page, it makes them approachable and customers like to feel like they are on even ground with the company they do business with. When they see Do Not Reply in every email, the message they receive is "we want to talk at you, but will not listen if you have anything to say."
With that said, if your social media plan is just to "build a Facebook fan page" or get on twitter, please don't. Your plan should be how you plan to communicate with your customers. Not just set a page, and forget it. Customers will either learn that you don't listen there either, or they will just forget you are there.
Just like with all advertising and marketing, why spend money telling people about your company if your potential customers will just feel disappointed once they find you.