Just realised that "Awoo!" is an excellent substitute for "Hello Fediverse!" That's ELEVEN characters saved. Think what you could say with eleven characters.

With 11 characters I can almost write my username.

11 characters is just enough for a your mum joke:
...
"So's yr mum"

You could also complement someone enthusiastically:
"Nice face!!"
--Thanks @cassolotl

Please respond with your own elegant uses of 11 characters!