The Press Complaints Council (PPC) dismissed civil servant Sarah Baskerville's complaint when personal information (including that she was hungover) originally shared by her with 700 followers on Twitter reappeared in newspapers. Unless the Twitter user sets specific privacy limits, individual Twitter messages, known as "tweets" can always be forwarded (or "retweeted") by their recipients. Accordingly, Ms Baskerville had no legitimate expectation of privacy with respect to her tweets.
This follows legal precedents which predate modern social networks. For example, in the law of defamation material which is only intended to be seen by the person whom it concerns (for example, a letter written to someone accusing them of a crime) cannot be defamatory. However, if one puts such material on a postcard (so others inevitably have access to it) it is taken to have been "published" for defamation purposes.
The moral is incredibly clear; if one wishes to keep material private, sharing it with 700 of one's closest friends via Twitter is not the best strategy to adopt!