Facebook's changes to the algorithms governing what appears in each user's news feed are worrying advertisers and agencies, who say the social network is forcing them to buy more ads to reach their "fans".
The social network is constantly trying to improve its best guesses for what users will find most engaging, while battling spam and "oversharing" from brands and friends alike. That is leading to some of the same complaints that have long plagued another huge, algorithm-driven source of web traffic: Google search.
Faced with daily invisible changes to Google's rankings and intense competition to appear below the most popular search queries, many marketers who once hoped that they would naturally rise up search results pages soon realised that guaranteeing visibility required investment in Google's sponsored results and search-engine optimisation tactics.
Now, many of those brands who attracted â€“ or bought â€“ millions of Facebook fans through its "like" button are now learning that their posts and photos are rarely seen for free, nudging them to buy its "Sponsored Stories" ads to cut through the noise.
Facebook says that the more clicks, likes, comments and shares that an item receives, the more users will see it. Its algorithm tends to highlight photos and videos over purely text-based posts, for example.
This "Edge Rank" system is designed to reward the most engaging pieces of content, whether a friend's wedding photos or a brand's video touting a new washing powder. Posts from brands and people are supposed to compete on a level playing field.
In a recent study, two social media marketing groups, We Are Social and Socialbakers, found a "significant decline" of about 50 per cent since August in the average number of fans a brand's Facebook post will reach. While Facebook said in May that 16 per cent of a brand page's fans will typically see each post, We Are Social found the average has fallen to around 12 per cent.
"EdgeRank has always worked in this way, but now it's been turbocharged and brands are competing for a much smaller 'share of newsfeed'," says Robin Grant, managing director of We Are Social.
Facebook said: "We're continuing to optimise the news feed to show the posts that people are most likely to engage with, ensuring they see the most interesting stories. This aligns with our vision that all content should be as engaging as the posts you see from friends and family."