Former Guardian editor: Facebook sucked up £20 million of our online ad revenue last year
Rusbridger, now principal of Lady Margaret Hall college at Oxford University, said The Guardian forecast online revenues of £100 million ($132 million) before he left in May last year.
In the end, the newspaper's digital turnover was £81.9 million ($108 million), which was down 2.3% on 2014.
Speaking at the FT Weekend Live Festival in London on Saturday, Rusbridger said The Guardian's prediction never materialised "because it all went to Facebook."
Business Insider has emailed The Guardian for comment. At the time it published its annual report in July, sources at the newspaper blamed online giants including Google and Facebook for hoovering up ad spend.
Rusbridger said Facebook does present some opportunities for publishers, but "they are taking all the money" because "they have algorithms we don't understand, which are a filter between what we do and how people receive it."
Rusbridger added: "This is going to get worse because they have a means of distribution which we simply can't cope with and the more people switch on to these devices, the more problematic that question is going to get."