Nov. 9 (Bloomberg) -- While Facebook and Twitter are popular sites for making friends, teens have also seen social media’s unfriendly side -- 88 percent of them report having witnessed mean or cruel behavior, according to a new study.

About 12 percent of the teenagers said they saw this type of behavior online “frequently,” while 29 percent said they observed it “sometimes,” said the report by the Pew Research Center, which surveyed 799 teens between the ages of 12 and 17.

At the same time, 69 percent of the teens said their peers are mostly kind on social-networking sites, the research said. About 95 percent of all American teens between the ages of 12 and 17 use the Internet, with 80 percent of them using social- media sites, the report said.

“Most of the time, these are pleasant places to be,” Amanda Lenhart, a senior researcher and lead author of the report, said in an interview. “But there are some dark moments popping up once a while. For a subset of teens, the world of social media presents a climate of drama and mean behavior.”

The research also said 12- and 13-year-old girls had the most negative assessment of social media. One third of them said people of their age are mostly unkind to one another on social- network sites, compared with 9 percent of boys in the same age group.

--Editors: Marcus Chan, Nick Turner

To contact the reporter on this story: Xu Wang in New York at xwang206@bloomberg.net

To contact the editor responsible for this story: Tom Giles at tgiles5@bloomberg.net