After analyzing more than 16,000 posts from retailers advertising on Facebook (NASDAQ:FB), a new report from social media analysis provider Expion suggests the first half of 2013 was the slowest growth period for Facebook advertising since 2011, led by a decline in both user engagement and the volume of ads, according to a press release Expion released today.

Expion's analysis found that luxury brands advertising on Facebook tended to have higher user engagement than other retailers, noting Tiffany and Victoria's Secret were the two most successful brands in 2013's first half. Additionally, the "quality" of a retailer's Facebook advertising initiative, according to the report, ultimately determines the success of its campaign rather than the volume of ads.

Video, Expion found, has yet to take hold for Facebook retailers, accounting for a "meager 3 percent," of all posted ads. Experion's report did note, however, that a reliance on images is likely to change as advertisers on Facebook, and other social media outlets, continue the transition to video.

Commenting on the report's findings, Expion CEO Peter Heffring was quoted as saying, "Key findings showcase that compelling content is still king, and brands that organically are tied to style and pop culture, like luxury brands, tend to benefit from the strongest engagement with their fans."