"It is unclear from your responses whether you believe any of the Russian-linked accounts involved in this influence campaign violated your respective user policies," Senator Dianne Feinstein said in a joint statement on Wednesday.
The letter says that Facebook failed to indicate if it had analyzed the possible links between Russian sponsors and the 2016 presidential campaign. Twitter also declined to answer the question of whether Russian sources had promoted the #Releasethememo hashtag.
The classified memo behind the #ReleaseTheMemo effort was prepared by Republicans and alleges that the FBI and the Justice Department had abused their powers by extending surveillance of an ex-Trump campaign associate amid the so-called Russian probe.
The House Intelligence Committee has earlier voted to make the memo public. The Republicans on the committee said that the release of the memo could shed the light on whether the Russian probe was biased in the first place. Texas Republican Rep. Mike Conaway said that the memo doesn't pose threat to national security.
Four separate investigations have been launched into the alleged Russian interference in 2016 US election, which saw the victory of Donald Trump.
Moscow has repeatedly denied the allegations of collusion, while Trump described the probe as the "single greatest witch hunt" in US history.
