Vietnam’s Facebook users — who now number 45 million, almost half the country’s population — use the site to organize prison visits and vigils outside police stations for detainees, and to solicit donations for political prisoners. And dissidents are increasingly migrating political and personal blogs, which can be easily blocked by the government, onto Facebook, which is so widely used that blocking it entirely would not be feasible.

Mr. Tuan helps run a fund that supports the families of prisoners of conscience, including Ms. Quynh’s mother and two young children. He said that much of the support now came from people inside the country sending money from their personal bank accounts, which the state can trace. In the past, he said, overseas Vietnamese communities drove most of the dissent and supplied most of the money.

“They know very well that they could be checked by the government, but they dare to do it,” he said of his local donors.

That has not gone unnoticed by the government, which is also asserting its authority in new ways. Ms. Quynh is one of over 100 bloggers and activists jailed in Vietnam, according to Human Rights Watch. Pham Minh Hoang, another popular blogger, was stripped of his citizenship and deported last week to France, where he also holds citizenship.

The government has strategically cut access to Facebook when protests are expected, and earlier this year asked both Facebook and YouTube to help it eliminate fake accounts and other “toxic” content, like anti-government material, saying it had identified up to 8,000 YouTube videos that fit that description, according to the local newspaper Tuoi Tre. The government also warned Vietnamese companies that their ads must not appear next to that sort of content.