I believe the most important thing is to thoroughly explain how your product will impact our lives. Try to explain everything, really. If your landing page is a bit long, put your call-to-action on top, then explain the benefits (asmentioned) and at the very end put again your cta. This is important because if you don’t do your bounce rate will be too high.
If your product is for businesses also, you can try to provide a pre-beta or beta subscription with a discount. You can usefor that.
Another thing to do it having referrals in mind from the very beginning. Don’t put share buttons everywhere. Just think of something that will add value to your users by referring your product (or give them something, either within your product or even an e-book with best practices, etc.).
Aswrote, Twitter is a very powerful tool for early traction. Install buffer on any of your devices in order to schedule your tweets and use for maximizing your reach by analyzing when your followers are online on Twitter. Auto-favoriting works – is good enough. However, the most important thing is to make friends. Retweet others, mention them and so on. Be frank and friendly. If you make friends and your product rocks, they will share it on their own.
Make sure that you focus on the loyal users who love your product and try to make them really happy. Then go for the rest.