The cost per impression of advertising on Facebook was 7X higher in November 2014 than a year earlier.

According to Kenshoo, which provided the figures, this is due to the move towards ads which as posts in users' Newsfeeds, rather than in the site's right-hand side bar – a change that is driven by mobile adoption.

However, return on investment (ROI) for Facebook's advertisers doubled over the same period – left in the dust by the rising costs, though Kenshoo says that this growth, coupled with a 5X increase in CTR (Click-Through Rate), still justifies the increase in costs.

The study also compares the cost and ROI of Facebook's Marketplace ads, which can be bought through the Power Editor platform, and Exchange, its RTB platform, on Monday 1 December – 'Cyber Monday'.

Compared to the same date last year, ROI on advertising bought through Exchange grew 5X – while the average CPM went up just 3X. For Marketplace, however, the 10X growth in CPM far exceeded the 5X increase in ROI.