Nearly half of the girls targeted had been threatened with physical or sexual violence, according to the poll. Many said the abuse took a mental toll, and a quarter felt physically unsafe.
"It is time for this to stop. Girls should not have to put up with behavior online which would be criminal on the streets," the report said.
Facebook and Instagram said they used artificial intelligence to look for bullying content, constantly monitored users' reports of abuse, and always removed rape threats.
"Keeping women safe on our apps is critical and we invest heavily in keeping abuse off our platforms," said Cindy Southworth, head of women's safety at Facebook.
She said Facebook was working with Plan International to better understand how it can support young women around the world.
Twitter said it also used technology to catch abusive content and has launched tools to improve users' control over their conversations.
The survey polled 14,000 girls and young women aged 15 to 25 in 22 countries including Brazil, India, Nigeria, Spain, Thailand and the United States.
Albrectsen said activists, including those campaigning for gender equality and on LGBT+ issues, were often targeted particularly viciously, and their lives and families threatened.
"Driving girls out of online spaces is hugely disempowering in an increasingly digital world, and damages their ability to be seen, heard and become leaders," she added.