Facebook and Twitter have been blocked in Russia by the country's communications regulator amid tensions over the war in Ukraine.
Roskomnadzor said its decision over Facebook was in response to what it claimed was restricted access to Russian media on the social media platform.
According to the regulator, there have been 26 cases of discrimination against Russian media by Facebook since October 2020, with access restricted to state-backed news outlets including RT, Sputnik, RIA, the defence ministry's Zvezda TV, and websites gazeta.ru and lenta.ru.
Roskomnadzor later said it also cut access to Twitter in line with a decision by the Russian prosecutor general's office.
The watchdog has previously accused the platform of failing to delete content banned by Russian authorities.
Reacting to the Russian decision over Facebook, the US said it was "deeply concerned" by the move, saying it was part of broader attempts to prevent its citizens getting information.
"This is part of their effort...to cut off a range of information from their public," White House spokesperson Jen Psaki said.
"We are deeply concerned about this and concerned about the threat on freedom of speech in the country."
Facebook's forced exit from Russia came hours after Microsoft announced of its own accord that it is suspending all new sales of its products and services in Russia, as did gaming giant Electronic Arts - the maker of the FIFA football series.
Meta runs Facebook, Instagram, and WhatsApp. Meta's president of global affairs, Sir Nick Clegg, said the company would continue to do everything it could to restore its services.
Reacting to the regulator's decision, he tweeted: "Soon millions of ordinary Russians will find themselves cut off from reliable information, deprived of their everyday ways of connecting with family and friends and silenced from speaking out.
"We will continue to do everything we can to restore our services so they remain available to people to safely and securely express themselves and organise for action."
Last Friday, Moscow said it was partially limiting access to Facebook, a move the company said came after it refused a government request to stop the independent fact-checking of several Russian state media outlets.
By Saturday, Twitter also said its service was being restricted for some Russian users.
Since Russia's invasion of Ukraine more than a week ago, tech giants have faced pressure to respond to the offensive, which the Kremlin calls a "special operation".
Governments have imposed economic sanctions on Moscow, and Meta said in recent days it had restricted access to RT and Sputnik across the EU and was globally demoting content from Russian state-controlled outlets' Facebook pages and Instagram accounts, and posts containing links to those outlets on Facebook.
Now Facebook and Instagram are blocking RT and Sputnik for users in the UK following a request by the British government.
It follows a letter by Culture Secretary Nadine Dorries in which she urged Meta, TikTok, and Twitter to restrict access to the Russian state-owned organisations, which she accused of spreading "damaging propaganda into Britain".
"I want to urge you to do everything you can to apply the approach you are taking across the EU to block access both to RT and to Sputnik's online output on your UK services," said Ms Dorries.
A spokesperson for Meta told Sky News: "Earlier this week, we announced that we'd be restricting access to RT and Sputnik across the EU."
"Consistent with that action, and following a request from the UK government, we will also be restricting access to RT and Sputnik in the UK at this time," they added.
In response to Meta's move, Ms Dorries tweeted: "I thank Meta in responding to my call to remove RT and Sputnik from their platforms in the UK. It is important that we do all we can to tackle propaganda pushed by Putin's Russia."
Announcing the EU's sanctions against "the Kremlin's media machine" on Sunday, European Commission president Ursula von der Leyen said: "The state-owned Russia Today and Sputnik, and their subsidiaries, will no longer be able to spread their lies to justify Putin's war."
Earlier this week, YouTube made the decision to block access to RT and Sputnik across Europe and in the UK at the same time.
Twitter and TikTok have not yet responded to Ms Dorries' request.