Social media has become a key forum for discussing US race relations -- but far more so for blacks than whites, a study showed Monday.

The Pew Research Center report found black social media users in the United States are roughly twice as likely as whites -- 68 to 35 percent -- to say that race relations are a part of their social networking conversations.

The report comes amid a growing social movement using #BlackLivesMatter and related hashtags following the deaths of African-Americans in police custody.

Among black social media users, 28 percent said most or some of what they post is about race or race relations but just eight percent of whites said the same, Pew found.

Meanwhile two-thirds of whites who use social media said that none of things they post or share pertain to race, the researchers said.

Some analysts give networks such as Twitter credit for popularizing social justice movements.

At the time of the social network's 10th anniversary this year, it noted that #Ferguson was the top social hashtag, emerging after the police shooting of an unarmed black youth in the Missouri town.

That was followed by #LoveWins, a reaction to a US court ruling in favor of same-sex marriage, and #BlackLivesMatter in third place.

"Social media also can serve as an important venue where groups with common interests come together to share ideas and information," the study authors wrote.

"And at times, Twitter, Facebook and other social media sites can help users bring greater attention to issues through their collective voice."

The researchers said the tone of social media conversations changed after the shootings this year of police officers in Dallas and Baton Rouge, Louisiana, which fueled a rise in the hashtags #AllLivesMatter and #BlueLivesMatter.

"There was a dramatic rise in the share of tweets criticizing the Black Lives Matter movement using that hashtag in our July analysis and a drop in the share of tweets that supported the movement," the researchers said.

"The rise in critical tweets was especially notable after the killing of police officers in Dallas."

The survey was based on interviews from February 29 to May 8 among a national sample of 3,769 adults, with a margin of error for the full group estimated at 2.3 percentage points.