Can I write whatever I want online?
The practical answer is no, you cannot write whatever you want online. Defamation law applies to material that you write. And given the worldwide reach of the internet, something you write online could become subject to the defamation laws of several countries around the world.
A fictitious example will help to illustrate the way defamation laws can apply to what you write online:
You are the father of a teenager who is addicted to the music of Generic Musician ("GM"). Your teenager plays GM's music at full volume day and night, and it's driving you mad. Out of frustration, you decide to take revenge on GM.
So you cook up a completely false story about GM and put it on your page on a social networking site. The story is that GM has stolen and plagiarised all of his music, including all of his biggest hits, from a brilliant yet obscure and unknown singer-songwriter who is so painfully shy that his rare public performances are given only in small pubs and clubs in his town.
Your fabricated story goes viral on the internet, and soon it's popping up all over the world. GM's managers take note of this, and also take note of the fact that the story seems to be driving down sales of GM's music and decimating ticket sales for GM's upcoming tour. GM's fan support in the US and most of Europe seems to be taking the worst hit. His managers are very worried by that, since the US and Europe are the areas in which GM is most popular and sells the most music. In fact, an online movement to boycott all of GM's music and concerts is even beginning to take hold.
GM's people trace the fabricated story to you, and having identified you are considering their alternatives.
Have you defamed GM? Under English law, as well as the law of many other jurisdictions, you have. Your story has injured his reputation and caused people to think ill of him.
Do you have a defence? Probably not. As the story is a complete fabrication, you can't defend on the grounds of truth. Your story is not fair comment either, as it was purely an attempt to discredit him (out of revenge for your teenager's use of GM's music to assault your senses). You were not offering an honestly held opinion about GM's music, but rather had concocted a story in the hope of causing him harm and annoyance.
Your story is not subject to any privilege or qualified privilege (although if the story were true, and you have simply reported GM to an intellectual property rights enforcement authority, then the qualified privilege probably would protect you).
Since GM is enormously popular in the UK, the harm that your story has done to his reputation could give rise to substantial damages – for which you would be liable if (as seems clear) your story was defamatory.
Where else could GM sue you?
Under European law, GM probably has the option to (i) sue you in England for the harm done in all European Union countries, or (ii) sue you in each of the EU countries in which GM has a significant reputation (which your story has harmed).
Moreover, given GM's popularity in the US, he could probably sue you there as well. And there are probably other countries where GM is popular and where your online story was seen. So there could be any number of places where GM could sue you for defamation.
What about the social networking site where you first posted the story?
If you have limited financial resources, GM's managers probably won't bother suing you. Instead they'll focus their fire on the organisations whose websites hosted the story.
As a first step, GM's managers will complain to the websites concerned, including the social networking site you used to first publish the story, and ask that they take down the offending material. If the websites comply in reasonable time, they'll probably have a valid defence to any defamation claim under English and European law. If they fail to comply, however, they should expect to face legal action for damages and/or an injunction (to take down the defamatory content).
Protecting yourself and getting help
The nature of the internet is such that a defamation problem can quickly escalate. Whereas journalists who work for large news organisations usually have ready access to legal advice before they publish, the individual blogger or product reviewer probably doesn't have that kind of advice at his disposal.
If you're in doubt about something you've written or intend to write online, you may want to do some research on your own to get a basic understanding as to how defamation law works. Reading the FindLaw articles on Libel and Defamation Law will help.
And if you've been accused of writing something defamatory online, you will probably want to get legal advice without delay.