I like to keep an eye on electric car sales. It's not the most fascinating thing in the world, but it's a kind of barometer for the green transportation sector.* So to recap a bit, not all plug-in manufacturers break out their sales per month - for example, Tesla only gives updates for each 3-month period, with Oct-Dec showing sales of 6,900 Model S EVs - while others like GM and Nissan give sales info for each month.
The full-year 2013 figures for those were 22,610 for the Nissan LEAF, more than doubling its sales from 2012 (9,819) and pushing it over 100,000 cumulative EVs sold, and 23,094 for the Chevrolet Volt.
December 2013 sales were 2,529 units sold for the LEAF versus the 2,392 units of the Volt. This is significantly higher than the January 2014 numbers, which are 1,252 for the LEAF and 918 for the Volt, but it looks like there's heavily seasonality involved with these numbers. I'm guessing that people want to get the tax credits before the year ends, so December sees more heavy EV sales, and slower sales in January because some people who might have waited until that month will buy earlier.
*But as we always say, the greenest way to get around is to walk, bike, or take transit. But that's not always possible for everyone (those of us who live in dense cities full of great transit infrastructure and bike lanes sometimes forget that half of world population lives outside of cities, and many of those who do live in cities are stuck in car-centric suburbia). That's why plug-in vehicles are so important.