Until December 2013, Justine Sacco, a 30-year-old PR woman from New York, liked to post jokes on Twitter.

Sacco only had 170 followers, so she would rarely experience interactions with others on the site. It must have felt like shouting into an empty room. A little unsatisfying, but without any real consequences.

However, something strange happened during her connecting flight from London Heathrow to Cape Town, where she planned to spend the holidays with her family.

One of her tweets went viral.

"Going to Africa. Hope I don’t get AIDS. Just kidding. I’m white!” she had posted just before turning her phone off to board the plane.

Enjoying a rare 11 hours of disconnect while in the sky, Sacco was oblivious to the world's reaction to her poorly-judged tweet. Without knowing, she had become the number one worldwide trending topic that night. People called for her to be sacked from her job, while others threatened to physically attack her. Another group, fascinated by the idea of someone having their life ruined online without being aware of it, started a popular hashtag: #hasjustinelandedyet.

The joke was meant to be a liberal mocking of white privilege, Sacco later explained to journalist Jon Ronson. That didn't matter. She quickly "parted ways" with her job and spent years depressed, tainted by the incident whenever anyone Googled her name.

Why Twitter's user growth is shrinking

According to Ronson, author of "So You've Been Publicly Shamed," this anecdote is indicative of a wider problem that can partially explain Twitter's declining user growth.

That third-party data shows the number of tweets per day created by Twitter's users has fallen by more than half since a peak in August 2014 — though Twitter does deny these figures are correct. However, its own numbers show that new user growth is stalling.

Speaking at Advertising Week Europe in London on Monday, Ronson spoke about how he had spent years following and interviewing people, including Sacco, who had had their lives torn apart by social justice mobs on Twitter. Twitter's power to cause the destruction of individuals could lead to Twitter's own downfall, Ronson said.

"I think the fact that Twitter is shrinking shows that when mean people take over social justice, people turn away from social justice," Ronson explained.

And Twitter has not been able to stop the "mean people" from dominating, he said.

"I don't think Twitter have addressed the problem at all," Ronson said. "They've been terrible at addressing the problem."

Twitter has made several efforts to tackle online abuse on the platform such as making the process of reporting bad behavior easier. In February this year, the company announced a "trust and safety council," — "a new and foundational part of our strategy to ensure that people feel safe expressing themselves on Twitter."

Twitter has turned into a place where "the hunt is on for shameful secrets"

According to the author and Guardian journalist, Twitter's atmosphere has turned significantly darker since it first launched:

"Twitter went from a place of curiosity, where people would admit shameful secrets about themselves and other people would say, 'Oh my God, I'm exactly the same,' to a place where the hunt is on for shameful secrets. We had the opportunity to go into other people's homes and be curious and compassionate and now we’ve gone into other peoples homes and we’re kind of instantly judgmental."

Ronson explained that although the online shaming comes from the decent aim of "egalitarianism and social justice," its effects are "appalling" when one person is singled out for wide-scale ridicule. There's something particular to Twitter that makes it worse for online shaming than other platforms like Facebook at Instagram, according to Ronson:

"Twitter does seem to be the worst. Facebook is bad too. I haven’t seen it happening anywhere near as much on Instagram and obviously it’s just harder for things to go viral on Instagram. Also maybe because ... a drone strike operator doesn’t need to think about the village he’s just blown up, but on Instagram you can often see the village you’ve just blown up [to a greater degree than on Twitter.]"（source：SCMP）