A lady in London was facing a libel action (HIGH COURT OF JUSTICE, QUEEN'S BENCH DIVISION. Claim number HQ12D05081 Mr Kirby Kearns and Mrs Lesley Kemp) after she complained about her employer on twitter. Lesley Kemp was paid £146 for her translation work by a company Resolution Productions. However, £25 were deducted as bank fees. Kempt rented on her twitter account and called the company 'disgraceful'.
Qatar based Resolutions Productions paid the £25, then sued her for libel over her tweets. They are seeking £50,000 in damages and £70,000 in cost.
Across the Atlantic Ocean, Amy Nicholls of South Carolina was sued (The Court of Common Pleas in Medina County, Ohio, Case No. 13CIV0351) by an Ohio-based company Med Express over a negative review she left at the company's eBay seller account. Nicholls was surprised by a postage-due for products she ordered online which she paid in full plus shipping fees, and left a negative review "'Order arrived with postage due with no communication from seller beforehand." on the transaction. Med Express admitted the comment was true, but nevertheless not happy it might hurt their business.
What was absent from the picture was eBay, the ecommerce giant where the transaction and user feedbacks took place. EBay set up a reviewing system as an essential and critical feature to remedy the uncertainty and unfamiliarity of an online auction place. If eBay did not back up its user who used the system exactly as it is supposed to work, then eBay literally set up a trap for its users. In this case, standing by is not only coward, unethical, but also suicidal.
So you could be sued for comments made on Twitter, or negative reviews left at eBay even though they are truthful and factual. You might find a lawyer who thought your case was significant enough that they would defend you pro bono. However, for every publicized cases with happy ending, there are many who have concluded the other way.