Codex is a company that performs book marketing studies based on surveys of book readers. The results of their latest study based on 8000 plus readers filtered out to the web in February but these results are still unknown to many people, so I will post the main points here.
The majority (81%) of book buyers found their books by what we can call "traditional means". These are things like browsing bookstores, recommendations from people they know, news, interviews, reviews, e-mail (yes, this one is now a "traditional means"), advertisements, etc. The remaining 19% found about their books from online sources. These sources are things like author websites, e-book stores, online advertisements, blogs, reader reviews, etc.
The above comes as no surprise. After all, print books are still about 80% of the books sold in the United States, and the reading and book shopping habits of people do not change overnight.
What was surprising is that of the 19% that found their books online, only 1.2% discovered the last book they bought through social networks like Twitter, Facebook, or video book trailers.
So to those authors out there putting hours and hours into their Twitter and Facebook accounts and trying to make a video promoting their latest book, the lesson should be clear. Consider devoting more of your time connecting with readers through blogs, reviews, or online advertising. Of course a little blurb in the local newspaper wouldn't be that bad either.