For the last year, American Express has been getting fantastic results by posting historical brand images to its Twitter, Facebook and Google+ pages. Think archeology or nostalgia as social media branding.

"We're focused on a number of things to keep engagement high across our social platforms," explained Mona Hamouly, vp of social media for the New York-based financial giant. "One of the ways we do that is by offering a wide variety of the type of content we post."

Not to mention a wide variety of photos from the brand's 163-year heritage in an initiative called #AmexArchive. Hamouly exclusively shared the following data points with Adweek that demonstrate how offering up grainy, decades-old pictures has created superb social engagement without any ad spend.

The numbers below represent how average engagement for the archive posts has continuously improved over the last several months.

On Twitter, a 160 percent increase in retweets

127 percent bump in Facebook likes

120 percent increase in Facebook comments

100 percent lift in Facebook shares

On Google+, a 104 percent hike in +1s

155 percent increase in Google+ comments

346 percent jump in Google+ reshares

On Instagram, a 112 percent lift in likes and comments

And here are examples from #AmexArchive: