Fergie wrote an open letter about the good and bad impact of apps including Twitter, Instagram and Facebook.
She said: “In so many ways, the internet has been a force for good.
“It enables us to keep in touch with friends and family all across the world.”
But it also has its downsides, she said.
Fergie added: “We need to pay more attention than ever to what we’re all being exposed to online.
“It’s time to confront head-on the fact that much of social media has become a sewer.
“There’s a large minority who seem to think that all the normal rules of society don’t apply to the internet.”
The #hellotokindness campaign has the support of MPs and celebrities