The perfect recipe for a viral social media campaign: Take a quirky idea, mix with a lot of patience, plenty of hope and a dash of serendipity.

This was what KFC's social media campaign on its Twitter account achieved with a quietly playful Twitter move that has gone viral.

The popular fried chicken chain followed the Twitter accounts of the five Spice Girls - Geri Horner, Mel B, Emma Bunton, Mel C and Victoria Beckham - and six random guys named Herb.

It was a play on its famously well-guarded secret recipe, which consists 11 herbs and spices.

Twitter user @edgette22 noticed this hidden detail on Friday (Oct 20), and tweeted about it.

He wrote:"Those 11 people? 5 Spice Girls and 6 guys named Herb. 11 Herbs & Spices. I need time to process this."

As of Sunday (Oct 22) the tweet has been retweeted 311,827 times and garnered 692,674 likes.

The little easter egg was planted "over a month ago", Mr Freddie Powell, creative director at advertising firm Wieden+Kennedy which dreamt up the campaign, told advertising trade publication Adweek.

The advertising firm is behind some other well-known ad campaigns, including Nike' s Just Do It tagline , a viral Super Bowl ad for Chrysler starring Eminem as well as Old Spice's The Man Your Man Could Smell Like.

Mr Powell added in his Adweek interview: "Frankly, we weren't sure if anybody was going to find it. Sometimes you just have to put stuff out into the universe and cross your fingers that the internet will work its magic."

Digital marketing manager for KFC US, Mr Bentley McBentleson, said playfully in a statement after the move went viral:"Our vault was getting cleaned so I thought the best place to keep the secret recipe was on Twitter."

"'No one's going to look at who we're following!' I thought. Boy was I wrong. I've made a huge mistake."