Also here's another site that has the ping system,
https://recolor.meSame system, just @ who you want to talk to, and to get notifications what they do is have a little closeable tool bar to the side. When closed it's a circle and doesn't get in the way.
It appears in my mobile and desktop versions.
When someones pings you, there's a purple @ symbol inside.
I'm pretty sure that wouldn't work too well or maybe the admins wouldn't be up to do the coding for that but they could force pop up notifications so that people on mobile also knkw