There have been a number of high profile cases such as the comedian Alan Davies, Sally Bercow and Peaches Geldof. They individually have made comments on legal proceedings which would be considered to be in contempt of court.
The BBC has created a useful guide to consider when updating your status or sending a tweet http://www.bbc.co.uk/news/magazine-20782257
Remember if you make a statement via any social media website it is in a public domain.