For the first time, the social network highlighted ad blocking technologies as a significant risk factor in its annual 10-K filing, noting that they have impacted its ad revenue "from time to time."

Although Facebook mentioned ad blockers in its 10-K last year, too, it did so only briefly as one of many things that could potentially impact its ads businesses, whereas it dedicated a whole section to the technology this time around. 

The warning about ad blockers comes as Facebook delivered blockbuster fourth-quarter financial results and as its ad business appears to be firing on all cylinders.