Twitter Inc. (TWTR.N) just seized a title no company wants: the worst performer in the S&P 500 Index.

Shares of the social-media giant dropped about 35 per cent in third-quarter trading, which ended Friday -- worse than every other company in the index.

Twitter’s tough quarter was set off by a disappointing earnings report. The stock plunged the most in four years after the firm said in July that monthly users had dropped by 1 million in the second quarter and predicted further declines as it fights against spam, fake accounts and malicious rhetoric. Shares sunk further earlier this month after an analyst at MoffettNathanson flagged “ smoke and mirrors” in the company’s expense reporting.

Short interest has risen to almost 8 per cent of float, according to data from financial-analytics firm S3 Partners. That compares with less than 5 per cent at the end of July.

Meanwhile, Twitter Chief Executive Officer Jack Dorsey’s other company has been on fire. Shares of Square Inc. closed at a record high Friday, finishing the quarter as the second-best performer in the Russell 1000 Index. The stock’s latest gain brings its total increase this year to 186 percent.

The worst quarterly performers in the S&P 500 after Twitter are IPG Photonics Corp. with a 29 per cent drop and Western Digital Corp. with a 24 per cent decline.