Ending a sentence with "just" is short for "just fck my sht up". https://knowyourmeme.com/memes/brendan-frasers-alimony-just-...

It's meant to convey a sense of defeat. So in this case the sentence would read as implying that the author wanted Mastodon to replace Twitter but that it's not able and that fact is causing her emotional pain.

twitterjust... might mean the part where you need to split your thoughts into spoon sized tweets...

"how's the article coming along?"

"hold on, I'm twitterjusting it"

It’s why I said “most”

It's progressing, almost there, not yet ready.