Dave, CFO
Thanks Sheryl and good afternoon everyone. Q4 wrapped up a strong year for Facebook. In 2014, our revenue grew 58% to approximately 12.5 billion dollars and we generated over 3.6 billion dollars in free cash flow. We are very pleased with the continuing growth of our network. In December, the number of people using Facebook on an average day increased by 18% compared to last year to 890 million. The daily number represents 64% of the 1.39 billion people use Facebook during the month. Mobile remains the primary driver of our growth. We ended the year with 1.19 billion people using Facebook on mobile in the month.
We also continue to see solid growth with Instagram, messenger and Whatsapp recently crossing 300 million, 500 million and 700 million MAU respectively. Turning now to the financials, all of our comparisons are on a year over year bases unless otherwise noted. In addition, as a reminder, our non gap measures excludes stock based compensation and the amortization of intangibles. Total revenue in Q4 was 3.9 billion dollars up 49% or 53% on a constant currency basis. Given how significantly exchange rates have continued to move, we anticipate that this currency head will increase in 2015. I will give more color on this later in the call. Ad revenue was 3.6 billion dollars, up 53% or 58% on a constant currency basis. Mobile ad revenue in Q4 doubled to 2.5 billion dollars or 69 % of ad revenue compared to approximately 1.2 billion dollars or 53 % of ad revenue last year. Desktop ad revenue was up approximately 1% despite the fact that overall desktop usage was down. In Q4, the average price per ad increased 335 % while total ad impressions declined 65%.
Similar to last quarter these price volume trans were primarily driven by the redesign of our right hand column ads which rolls out in the third quarter. Total payments and other fees revenue was 257 million dollars, up 7 %. Note that the growth was driven from revenue by acquisitions made in the past year. On an organic basis, payments revenue from games, which represents the substantial majority of our payments and other fees revenue declined 10% compared to last year. As previously noted, we expect this trend to continue as desktop usage declines. Turning now to expenses, our Q4 total gap expenses were 2.7 billion dollars, up 87 % and non gap expenses were 1.6 billion dollars up 50%. Gap expense growth was driven primarily by significant stock base compensation and amortization expenses related to the Whatsapp acquisition. Non gap expense graph was driven primarily by increases in head count related cost, cost of revenue and marketing expenses. On a full year basis, our 2014 gap expenses were 7.4 billion dollars, up 47% and our non gap expenses were 5.3 billion dollars, up 34%. We ended the year with roughly 9200 employees, up 45%.
Overall we remain very pleased with our ability to attract and retain top gear talent. Gap operating income was 1.1 billion dollars in Q4, representing a 29% operating margin, down from 44% last year, again primarily due to expenses related to our recent large acquisitions. Non gap operating income was 2.2 billion dollars in Q4 representing a 58% operating margin, consistent with the margin last year. Interest in other income and expense, was a net expense of 19 million dollars in Q4 versus a net expense of 3 million dollars in Q4 last year. This increase in expense was primarily due to foreign exchange losses resulting from the periodic re-measurement of our foreign currency balances during the period. In Q4, we benefited from the reinstatement of the R&D tax credit. Our gap tax rate was 37%, but would have been approximately 42% excluding the benefit of the tax credit. Our Q4 non gap tax rate was 31% and would have been approximately 32% excluding this benefit.
