(Adds details from CEO Dara Khosrowshahi's email to staff; Updates shares)
May 13 (Reuters) - Uber Technologies Inc's shares fell as much as 12% on Monday, more than doubling their losses since the ride-hailing giant's poorly received Wall Street debut on Friday and raising more questions about investors' faith in its ability to make profits.
The fall in shares come against the backdrop of a global stock market selloff sparked by renewed trade tensions between the United States and China.
The stock hit a low of $36.58, valuing the company at about$14 billion less than the IPO price of $45. Shares of smaller rival Lyft Inc, which went public at $72 a share on March 29, was down 7.3% at $47.38.
The stock "did not trade as well as we had hoped post-IPO", Chief Executive Dara Khosrowshahi wrote in a memo to employees that was seen by Reuters.
"Today is another tough day in the market," Khosrowshahi said, citing examples of Facebook and Amazon as their shares had initially struggled after going public.
"As the market sees evidence, sentiment will improve, and as sentiment improves, the stock will follow."
Uber lowered its valuation expectations twice in the past two months to address investor concerns over its mounting losses, and finally priced its IPO at the low end of the targeted range in a bid to avoid Lyft's stock market struggles.
"In the last couple of weeks we have noticed investors questioning more about how good of a business model is ride sharing really," said Tom White analyst D.A. Davidson.
While both Uber and Lyft are trying to find ways to lower driver costs to become profitable, drivers went on a protest in several U.S. cities earlier this month demanding job security, livable incomes and a cap on the amount ride-hailing companies can collect from fares.
Many investors are concerned about rising costs associated with booking fees shared with drivers, said Daniel Morgan, a senior portfolio manager at Synovus Trust.
Morgan said this expense will only rise as the company increases sales and demand for drivers grow.
Investors have struggled to figure out how much Uber and Lyft are worth, given both companies have not estimated a timeline for turning a profit.
Lyft posted a $1.1 billion quarterly loss last week and forecast losses would peak this year as it controlled expenses and got more revenue from each customer.
Uber has warned in a regulatory filing that it may never be profitable.
Investors are questioning whether achieving profitability will require these businesses to either raise prices for consumers or reduce service levels, White said.
Wedbush analyst Ygal Arounian said investors need to be patient as Uber reaches full monetization potential with its ride-sharing platform and a broader growth engine with Uber Eats, Uber Freight and autonomous driving initiatives.
"While it will take time for the stock to settle and Uber must execute flawlessly over the coming 12 to 18 months, we believe a $100 billion+ market cap is warranted," said Arounian, who has an "outperform" rating on the stock. (Reporting by Supantha Mukherjee and Vibhuti Sharma in Bengaluru; Additional reporting by Joshua Franklin in New York; Editing by Patrick Graham, Shounak Dasgupta and Anil D'Silva)