Three months after its flotation on the New York Stock Exchange, Twitter has reported a net loss of $US645 million for 2013.
The BBC says the loss was expected by analysts, who highlighted Twitter's revenues, which rose 110% last year to reach $665 million.
But a reported slow growth in user numbers was a bigger concern for investors.
Twitter averaged 241 million monthly users in the last quarter of the year, up 3.8% on the previous quarter.
The BBC says that represents a slowdown compared with a growth rate of 10% seen at the beginning of 2013.
Timeline views were down nearly 7%, suggesting users were refreshing their feeds less often.
More than 90% of its revenues in the last quarter came from advertising.
Shares fell as much as 12% in after-hours trading on Wednesday.
Twitter's share price has more than doubled in value since the company was floated on the stock market in November, when it was valued at around $US18 billion.