Kimberley Swann said she was not given a chance to explain her remark
A 16-year-old girl from Essex was fired after she described her office job as "boring" on her Facebook page.
Kimberley Swann, 16, of Clacton, had been working at Ivell Marketing & Logistics, in Clacton, for three weeks before being fired on Monday.
"I think they've stooped quite low," she said.
The firm's Steve Ivell said of the decision: "Her display of disrespect and dissatisfaction undermined the relationship and made it untenable."
Miss Swann said: "You shouldn't really be hassled outside work. It was only a throw-away comment.
"I came home from work one day, sat on the computer and said something about my job being boring."
Details were passed to her employers after she allowed colleagues access to her page, Miss Swann said, adding that she was not given the chance to explain.
Her mother, Janette, 41, said: "I think she's been treated totally unfairly. She didn't mention the company's name.
"This is a 16-year-old child we're talking about. She says Clacton is boring but we're not going to throw her out of the house for it."
Mr Ivell said: "Ivell Marketing is a small, close-knit family company and it is very important that all the staff work together in harmony.
"Had Miss Swann put up a poster on the staff notice board making the same comments and invited other staff to read it there would have been the same result."
TUC general secretary Brendan Barber said employers needed "thicker skins" in relation to social networking websites.
He said: "Most employers wouldn't dream of following their staff down the pub to see if they were sounding off about work to their friends."