Apparel groups have social opportunity

The equivalent figures stood at 684,000 and 22.2m in turn for La Redoute, the French fashion catalogue specialist, in its home market.

These extended groups offer brands the chance to start a "network effect", beginning with "likes", check-ins and comments by their fans, the study said.

"If the posted information appeals to their friends ... and they subsequently like the post it can cause a chain reaction of communication," it added. "This cascade of information means that brands can not only speak to their most loyal fans but also engage with their wider social network."

Similarly, paid-for impressions from TopShop, the retail chain, aimed solely at its Facebook fans secured a 1.6% reach across the entire UK web audience, rising to 2.3% when ads were targeted at their friends as well.

Totals on this measure came in at 1.4% and 2.8% for Zara, a unit of Inditex, in France. The same brand registered returns 0.7% and 1.2% respectively in Germany.

Upon considering unpaid brand impressions, the study used gross ratings points, or the share of fans and their friends reached, multiplied by the number of times they saw it. ASOS recorded the highest score of 34.2 points in the UK.

By contrast, H&M secured only 4.7 points on this metric in Germany. Lower frequency levels played a key role here, as the company had double the amount of "fans and friends" as ASOS, but the cohort consumed ads comparatively less often.

Finally, some 1.7% of a sample of Facebook users that were exposed to paid-for ASOS ads went on to make a purchase from its site in the next four weeks, versus 0.7% among those not exposed to these messages.