PHNOM PENH, Cambodia — Thy Sovantha, Cambodia’s youngest political star, is still seething.

A charismatic student, she rocketed to fame during the 2013 elections when she backed the opposition’s effort to unseat Cambodia’s authoritarian ruler. Her Facebook page drew hundreds of thousands of followers, making her, at 18, perhaps the most powerful voice of her generation.

The opposition nearly won the election, but protests over the vote led to a government crackdown. By November, the political battle had come down to a single standoff: The government threatened to arrest the opposition leader, Sam Rainsy, if he set foot in Cambodia again. Mr. Sam Rainsy, who was traveling abroad, vowed to return.

Ms. Thy Sovantha put out a call on YouTube and rallied hundreds of her supporters to meet him at the airport.

But at the last minute, he canceled his flight and fled to France.

“I was very angry,” Ms. Thy Sovantha said. “The reason we supported him is that we want change. If he does not come back like this, we think, how can we change the leader? How can we win?”

The question is a riddle in Cambodia, which has been stuck in roughly the same political cycle for decades.

For 30 years, Prime Minister Hun Sen, a former Khmer Rouge fighter, has wielded power through a combination of threats, clever deal-making and sheer willpower. And for most of that time, Mr. Sam Rainsy, a French-educated former finance minister, has been his foil. Some commentators here compare the pair to Tom and Jerry.

Mr. Hun Sen, who met with President Obama at a regional summit meeting in California this week, tolerates periods of relative freedom and political dissent to a point, but resorts to coups, crackdowns and court cases when serious challenges arise.

Mr. Sam Rainsy is now as well known for fleeing the country in the face of legal threats as he is for his reform-minded agenda. His retreat to France was his third in a decade.

Two-thirds of the population is under 30, making Cambodia one of the youngest nations in Asia, according to United Nations estimates. The first generation to grow up after the horrors of the Khmer Rouge regime faded and the economy began to grow at a rapid clip, they are better educated and more skeptical than previous generations. Increasingly, they get their news online rather than from traditional television and print news media, which are dominated by the governing party.

Politically, they are restless, having outgrown the authoritarian style and patronage system of Mr. Hun Sen.

But having come so close to ousting him and failed, many are disillusioned. And now some are starting to give up on Mr. Sam Rainsy as well.

“I feel disappointed and hopeless,” said Ms. Thy Sovantha, now 20.

His decision not to return to Cambodia, analysts say, was a lost opportunity, if not a surprise.

“The biggest threat to Hun Sen’s grip on power would be Sam Rainsy in jail,” said Ou Virak, the founder of Future Forum, a research institute. “The international condemnations and the potential closing of the American market in the garment industry, that would be nerve-racking for businesspeople.”

But even his supporters were disappointed.

Ou Ritthy, 28, the founder of a youth political discussion group, said he and his peers were exasperated with the cat-and-mouse game of Cambodian politics.

“Hun Sen, many things he has been doing are for power, for party interests, and Sam Rainsy also does the same,” he said. “It’s old-style politics in a new society.”

Mr. Ou Ritthy credits the sharp rise in Internet penetration and smartphone use for changing the dynamic here.

“Youth have two things,” he said. “Information — they got informed from social media — and smartphones. They are more independent in terms of information. They are not told what to do by their parents like in the past.”

After two opposition lawmakers were dragged from their cars and severely beaten in October, Facebook sleuths managed to identify several attackers as members of government security forces.

Both parties are keenly aware of the demographic shift and are trying to chase the changing electorate.

In his absence, Mr. Sam Rainsy has led town halls via Skype and shared political commentary and vacation snapshots with his fans on his widely followed Facebook page. Shirtless photos of him piloting watercraft at a luxury resort in the Philippines inspired heated debates on his character.

Mr. Hun Sen joined Facebook in September and has taken to it with a vengeance. He is particularly fond of posting candid snapshots of himself — sitting on the ground slurping up street noodles, swathed in a medical gown receiving a checkup, watering plants in a public garden and even taking selfies during a regional summit meeting.

According to a study by the public relations firm Burson-Marsteller, he already ranks second among world leaders for engagement with Facebook followers.

He is increasingly conducting government business on Facebook. He has made several policy changes based on complaints posted to his personal page, and last week, he announced the creation of dozens of “Facebook working groups” to gather information about citizens’ concerns.

Sun Heng, a 22-year-old university student, said he and most of his friends followed both leaders on Facebook but were still skeptical about the sexagenarians’ embrace of social media.

“For me, I find it very pretentious: Sometimes it is so obvious that they are acting,” he said. “But it can also be a good sign, showing that they care what people think now.”

“This is for me the fourth time that I am forced into exile,” he said by Skype from his home in Paris. “But eventually, each time, the situation changes, and I always manage to be back in time.”

The question for Cambodia is whether his followers will still be waiting. Ms. Thy Sovantha seems to have moved on.

She has told her followers, who now number 1.2 million, of her disillusionment with Mr. Sam Rainsy. Her Facebook posts focus increasingly on education and environmental issues rather than party politics, and she is channeling her energy into starting a center for homeless youth.

But she may not have entirely given up on the system. On her Facebook profile, she describes herself as “politician.”