Facebook has warned advertisers that it may not be able to accurately measure the success of their adverts due a privacy clampdown by Apple in iPhone software. The social media giant has added a disclaimer to its return on investment figures, which are crucial to advertisers' confidence their money is well spent on its platform, saying they might now be missing data or partly based on "statistical modelling". Advertisers told The Daily Telegraph that since Apple updated iPhone software last week, Facebook has provided unpredictable results that are much higher or lower than normal. One called the figures "nonsense", while another described the system as "broken". The change shows how Facebook's $84.2bn (£60.1bn) advertising operation could be affected by Apple's move, which the social network has said will damage millions of small businesses that rely on targeted adverts to grow.