NEW YORK (Reuters) - The trillion-dollar market cap club expanded last week to a third U.S. company, with Google parent Alphabet Inc GOOGL.O topping the lofty valuation mark. Adding the next member, however, is likely to take a while.

FILE PHOTO: The logo of Facebook is seen in Davos, Switzerland Januar 20, 2020. Picture taken January 20, 2020. REUTERS/Arnd Wiegmann

Apple Inc AAPL.O and Microsoft Corp MSFT.O are also worth more than $1 trillion on the stock market. And while Amazon.com Inc AMZN.O is currently worth around $935 billion, the e-commerce leader did cross the trillion-dollar threshold briefly in September 2018.

The next closest company is not within $300 billion of the mark, but a handful companies could contend based on their current values.

Social media platform Facebook Inc FB.O appears to have the pole position. Its $630 billion market value is about $65 billion more than Berkshire Hathaway Inc BRKa.N, the next richest company in the S&P 500 .SPX.

But a lot can happen between now and $1 trillion, and investors asked to handicap the field revealed different choices among the S&P 500 .SPX for which U.S. company will be the fifth member of the elite club. The next to join also may not yet be on anyone's list.

Graphic: Market-cap heavyweights (interactive)

Graphic: Market heavyweights

The following are among the top contenders to crack the $1 trillion market valuation mark:

FACEBOOK (Current market cap: $627 billion)

The social media company’s stock price soared about 50% in the past year, and while past performance is no predictor, Facebook would top $1 trillion in just over a year if it keeps up that pace.

To get there, Facebook “must demonstrate the ability to monetize its user base beyond just selling ads,” King Lip, chief strategist at Baker Avenue Wealth Management, said in an email.

Greater scrutiny of tech companies over privacy and data policies could limit Facebook’s growth and stock valuation, according to investors.

Facebook is poised for above-market growth, said Margaret Reid, senior portfolio manager at The Private Bank at Union Bank. But “that trillion dollar market cap might be further out the spectrum because of the regulatory environment for these technology platform companies,” she added.

BERKSHIRE HATHAWAY ($561 billion)

Shares of Warren Buffett’s conglomerate climbed more slowly than the broader market over the past few years, with its Class A shares up 11% in 2019 against a gain of nearly 29% for the S&P 500.

Berkshire’s “growth rate is less exciting, their valuation less likely to expand meaningfully,” according to emailed comments from Jason Ware, chief investment officer with Albion Financial Group, who called the diversified company’s earnings power and economic competitive advantages “airtight.”

“They’ll likely hit $1 trillion eventually, but they are the tortoise in this story.”

VISA ($446 billion)

Soaring stock prices have propelled Visa Inc V.N and rival payments processing company Mastercard Inc MA.N up the market cap chart.

Both could surpass Facebook in market value three years from now, if all three companies maintained their average annual stock price increases of the past three years.

With Visa shares trading at over 30 times forward 12-month earnings estimates and above their 10-year average market premium, according to Refinitiv Datastream, some investors are wary about its valuation.

Albion’s Ware said Visa’s dominance in the credit and debit markets “has afforded them highly visible recurring revenues and earnings and thus a stable market premium.”

If Visa’s P/E ratio stays stable or expands, Ware said, the company could reach $1 trillion “before many might expect.”

JP MORGAN ($428 billion)

JP Morgan Chase & Co JPM.N ranks as the largest U.S. bank by market value by well over $100 billion. Paul Nolte, portfolio manager at Kingsview Investment Management, said in an email that the bank could grow even bigger.

“There will be some legislative issues with a very large JP Morgan,” Nolte said. “But continued consolidation within the banking sector along with international competition could push the large to get even larger.”

Graphic: Top U.S. companies by market cap