A group of 50 Vietnamese activists and rights organisations have written an open letter to Facebook chief Mark Zuckerberg suggesting his company may be colluding with communist authorities to scrub out online dissent.

Vietnam ranks among Facebook's top 10 users by numbers and the site is hugely popular among dissidents in a one-party state where independent media is banned and blog sites are routinely removed.

The letter to Zuckerberg comes amid deepening controversy at Facebook over privacy and security lapses after the revelation that British firm Cambridge Analytica -- which worked with Donald Trump's campaign -- hijacked data on millions of users.