The avalanche of painful personal stories comes amid a series of high profile sex abuse scandals.
More than 27,000 people replied, making "#MeToo" the top trending topic through the day Sunday.
The avalanche of painful personal stories comes amid a series of high profile sex abuse scandals -- the latest one centered around powerful Hollywood producer Harvey Weinstein.
The intensity and breadth of the response highlighted that the problem reaches well beyond the rich and famous and features in the everyday lives of women around the world.
"If you've been sexually harassed or assaulted write 'me too' as a reply to this tweet," Milano wrote.
Many women appeared to be speaking out for the first time about abuses they had suffered, and said they had to overcome feelings of shame and embarrassment to do so.
Some just responded "#MeToo."
Others said they had been abused as children by relatives, or as a teenager by a person they trusted. That nobody believed them when they spoke about it emerged as a common refrain.
"I wish I could remember who I was before #MeToo," wrote a woman named Rosey.
"Molested by a family member. Raped as a kid and an adult. Became a drug addict and then overcame. Don't ever give up. I'm here#MeToo," said Amy Christensen in her post.
"Sexual assaulted by a military doctor at Lackland AFB. 1973," said DebiDay.
"Me, too. I spoke out. What did I learn. That no one, absolutely no one, would listen, much less help," said Lisa Omlid.
"If you are not at the point where you can share your #MeToo I stand with you," said Kelly Douglas. "Your story is valid whether you share it or not."
There were many sympathetic responses from men.
Vinay Ramesh encouraged "all my fellow men to learn about #MeToo. The responsibility to stop sexual violence is absolutely on us."
"I feel disgusted and sad," said Ed Krassenstein.
Although the overwhelming majority of the posts were personal in nature, the Weinstein scandal and US President Donald Trump's videotaped boasts of groping women with impunity also drew comments.
"Just as I did after the #Trump tapes, I'm having flashbacks due to the Harvey Weinstein assault & rape allegations. Abuse is forever," wrote BuffieHippie.
More than 20 women -- a who's who of Hollywood -- have come forward to accuse Weinstein of rape, assault and sexual harassment.
Milano, probably best known for her role in the 1980s sitcom "Who's the Boss," is not among the accusers.
Weinstein, who insists any sexual encounters were consensual, was expelled this week from the Academy of Motion Picture Arts and Sciences.
A court document made public on Sunday revealed that Trump's campaign has been subpoenaed for all documents related to assault allegations that emerged during his run for the presidency.
The subpoena is part of a defamation lawsuit brought by Summer Zervos, a former contestant on Trump's reality TV show "The Apprentice" who alleged he tried to kiss and grope her without her consent.