Perhaps as a marketer I am biased, but I think the most important thing to know when you are launching your business is your customer. Without a solid understanding of your customer, your business probably will not survive. With a good understanding of your customer, you can meet and even exceed their expectations at every touch point.
Your customers are the people who will vote for (or not vote for) your products with their wallets. If you have a good understanding of your customer, you will be able to build a great product or service and tailor your experience to meet the needs of the people who shop with you.
Once you understand who your customers are and their preferred communication style, you only need to master that single communication style. If your clients or customers completely ignore Twitter, you don't have to waste your valuable time on that platform. Likewise, if they spend a lot of time interacting on Pinterest, your brand should be represented on that platform by pinning, creating groups, possibly by sponsoring pins. You can learn this information about your customers by asking them. Ask the people who currently buy your products if you've already launched. If not, ask questions of the people who you believe will ultimately be your main customers. If you've put up a lead page to collect emails, you can send those people an email to ask about their preferences. Ask them about products that they currently buy and about products that they wish were available. Finally, ask them about their communication style. You can use this information to create the best products possible and to find the best ways to send them your brand messages.
Your interactions should be a positive experience for your clients both online and off. The best way to ensure that is by understanding their habits, communication style, and what they want from you.