If a Twitter user posts an illegal tweet, they could get sued, but Twitter could be sued itself as a secondary publisher according to Zack Whittaker. Legal analysis site Out-Law published a very interesting, theoretical piece, which 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.
A case of mistaken identity in Australia illustrates the point. Someone wrote a hateful blog about writer and television personality Marieke Hardy. She wrote a blog post accusing Joshua Meggitt of being its author and used her Twitter account to draw attention to her post.Hardy was wrong to finger Meggitt as the author of the original material and she reportedly paid Au$15,000 (£10,000) to settle the case. Will Twitter still be held liable for the libel?
One more thing, Those who retweeted are not being pursued. 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. This means that Twitter and other social media platforms can be held liable for defamatory material published by a user.
[Source 1 & Source 2]