According to the emails, published here on the SEC's website, Facebook's initial filing reported Zynga as contributing around 12 per cent of the social network's 2011 revenue - but further emails revealed that it was substantially higher; Facebook had omitted the advertising revenue it reaped from Zynga apps and so had to revise the figure upwards to 19 per cent of revenue. According to the emails, Facebook also dragged its feet over disclosing just how many people accessed the site through mobile devices. One of the most controversial aspects in Facebook's IPO was its struggle to monetise mobile access - which is how, increasingly, people engage with the site.