Who you are online is part of your brand. The tweets you send out. The Facebook updates you post. All of these attract followers and sometimes your brand is built around you without you knowing.
Think about it: you’re just being you online, but the people that follow your thoughts are attaching themselves to you and these are the people you are projecting your message to when it comes time to promote your book. On Twitter the people you follow aren’t necessarily the people that follow you–so when you’re trying to grow online remember to think about your following and those that are interested in your message. These are the people that are going to buy your book if you market it correctly to them.
The more information you know about your Twitter followers and Facebook friends (i.e. by engaging with them!) the better you can pitch your message and make it successful (i.e. at book promotion time). People want authentic engagement, so bring your followers the relationship building experience and the product they can care about (and buy!).