maybe they have not been interested and don't need it, they still feel enough with what they have achieved right now, if they knew how much they would benefit from ico or this technology, I think they would join this platform without thinking.
They are already established social media, we can even call them giants already so there's no need for them to conduct any ICO. As far as I know those who conduct it are project startup companies. And I don't think that they're fully supported crypto as well. Just look at how Telegram becomes, there are hundreds of thousand of bots operating and its possible that if those aforementioned social media joins crypto they will be overrun by bots and its going hard for them to stop.