Legal analysis site Out-Law describes how Twitter could fall foul of the law through no apparent fault of its own but by giving its users free reign over what they say. Under the country’s defamation laws, internet service providers (ISPs) and internet content hosts (ICHs) are publishers and can be sued in relation to material published by a third party. Twitter and other social media platforms can be held liable for defamatory material posted by a user. Those who retweeted are not being pursued.
Source: https://thehackernews.com/2012/03/twitter-as-secondary-publisher-could-be.html