Establishing an online presence isn't just preferred if you want to be a successful writer; it's absolutely necessary. As the primary form of marketing, it's important to make yourself and your work known long before you get your first agent and contract. Publishers want authors that stand out from the rest: ones who not only have good writing skills but are marketable as well. And while the traditional forms of marketing and publication still lie with the publishers and agencies, it's up to the author to connect with others outside of the publication world and establish a name for themselves.
Smwright has an awesome post regarding the subject that I strongly encourage aspiring authors to check out:
http://smwright.wordpress.com/2013/11/06/the-business-of-writing-part-ii/
It talks about ways in which you can further your online presence and the key factors in doing so, including a website, professional email address, and various social media sites.
The biggest part of selling your work is to first market yourself!