Would you plan a party and not invite anyone? Without any guests, it wouldn't be a very fun party. The same goes for your website launch. Your goal should be to maximize awareness and keep your users participating. Launching a website may seem like the end of a process, but it's really just the beginning.
Email marketing is a great way to spread your message, but is only practical with a healthy list. The privacy issues tied to purchasing email addresses are serious, so growing your list in an organic and innovative fashion is crucial.