I feel for the victims of rape who do not get justice, but I feel a private facebook group you can join on facebook/twitter, where accounts can be set up with ease under false names opens such a system up to abuse by people who want to hurt people by spreading lies.
My ex told people I hit here after we split up and nobody could understand why we had when I was "such a nice guy". The reality of the situation was she didn't want to admit to family that we split up because she had started sleeping with too other guys and thought the grass was greener! thankfully this was before facebook was as popular as it is today (MSN messenger was the thing to communicate on.....man I'm old) and she got called out publically (at a party) by here best friend and cousin who were sick of her lies! I only found out she'd been telling people that when I bumped into her cousin after the fact.
This girl had issues, when we were together she (made up) told me and her best friend she was molested by her cousin, but we couldn't tell anyone! She told us this about a month after two of her cousins came out as being molested by a babysitter, and she (as the youngest and previously doted upon member of the family) was no longer centre of attention.
I believed her, her friend did not and swore me to keep my word not to say anything. She was confronted about this about 6 months later and claimed to have no idea what we were talking about, before eventually admitting she made it up as she felt left out! She was 20!
People like that exist and are exactly the type of person who probably already make shit like that up about people, giving them a dedicated public forum could cause all sorts of problems.
And that's just one scenario.
People are innocent until proven guilty and providing a vigilante way to without evidence state someone is violent/a rapist etc is defamation and utterly wrong.