This youtube video (created as part of a series of short programmes for the UK's Channel Four television called Susie Dent's guide to Swearing) claims (around 3:08 in) that the word "fuck" makes up about a third of all swearing on the internet.
Is there a source behind this estimate? Is is just a guess or has someone actually done some research?
PS what a joy to live in a country that has a TV programme called Susie Dent's Guide to Swearing!