If you have an existing website that is not mobile-friendly, you are effectively locking your front doors to large and growing percentage of web visitors. "Large" is not very descriptive, so I'll have to elaborate.
In 2013, about 14% of all Internet traffic worldwide is from mobile device. This mobile web use statistic grows in leaps each year. In 2010, mobile devices accounted for just 1% of web traffic. eMarketer.com estimates that in 2013 about 15% of all U.S. retail purchases
will be made from a mobile device, which is an increase of 4% over last year.
04 August, 2013