Alwaleed, a nephew of Saudi Arabia's king and estimated by Forbes magazine to be the 26th richest person in the world with a $19.6 billion fortune, already owns a 7 percent stake in News Corp and plans to start a cable news channel.

The purchase is remarkable because Twitter was a key means of communication for protesters in the Arab Spring revolts this year, violence that threatened Saudi Arabia until the kingdom unveiled a populist $130 billion social spending package.

Twitter, which allows people to send 140-character messages, or Tweets, to groups of followers, is one of the internet's most popular social networking services, along with Facebook and Zynga.

The Twitter stake, bought jointly by Alwaleed and his Kingdom Holding Co investment firm, resulted from "months of negotiations," Kingdom said.

Bernhard Warner, co-founder of analysis and advisory firm Social Media Influence, said: "The Arab world, of course, knows full well the value of Twitter. In the past year, it has been a force in politics, in regime change, so there is not a single person in that region in a position of influence who is not following the increasing power of Twitter.

"(Alwaleed) must see Twitter as something that is going to be a really powerful broadcast channel," he said, adding the Saudi had got into the internet boom belatedly, with mixed results, and appeared to be "kind of late" to the game again.

Investors in Saudi Arabia were more bullish, sending shares in Kingdom up 7 percent to 8.40 riyals.

"One of the few sectors to record significant revenue gains in the last three years has been technology, which is why Kingdom would see Twitter as a good addition to its diversified portfolio," said Hesham Tuffaha, head of asset management at Bakheet Investment Group in Riyadh.

Saudis are increasingly turning to satellite television, online news providers and social networking to stay abreast of world events. The world's No. 1 oil exporter announced a series of stricter regulations for journalists earlier this year.

Alwaleed, who has a sizeable stake in Citigroup, has spoken in favour of broader political participation, fair elections and effective job creation across the Arab world.

IPO HOPES

Investors are eagerly anticipating an initial public offering from Twitter, which said in September it was in no hurry to go public. It raised $400 million in venture capital financing this summer.

It now counts more than 100 million active users who log onto the service at least once a month. Facebook, the world's largest social network has more than 750 million active users.

Internet search giant Google recently launched a social networking service dubbed Google+ which some observers say could lure users away from Twitter.

Shares in online games developer Zynga ended at a 5 percent discount to their issue price on their trading debut on Friday, and analysts said any valuation for Twitter could be misleading.

"You could put any number of zeroes behind a valuation of a private company. Before it goes public, it is almost meaningless," said Warner.

"This is a very small group of investors which has put money into this thing. That will be diluted and diluted and diluted again until it goes public. And that is when we will see what the value is. These are kind of magic numbers at the moment."

Kingdom owns a near-30 percent stake in Saudi Research and Marketing Group, which runs a range of media titles.

"Our investment in Twitter reaffirms our ability in identifying suitable opportunities to invest in promising, high-growth businesses with a global impact," Alwaleed said.

Alwaleed subscribed $500 million to last year's General Motors IPO. In August, he unveiled plans to build the world's tallest tower in Jeddah.

(Reporting by Sitaraman Shankar; Additional reporting by Georgina Prodhan in London and Matt Smith in Dubai; Editing by Erica Billingham and Dan Lalor)

• A Permanent Economic Emergency