It seems clear the English speaking web has a preference for Bluesky. It would be interesting to know how much variation there is between users of other European languages. It seems to me the Germans are pretty active in the Fediverse, which makes sense considering a significant portion of them have been huge privacy nerds since the fall of the GDR.
Those reaction emojis ± count like discord vs upvote/downvote is like going from Grayscale to Color TV. Worlds Apart. Night & Day. Misskey is leading the way.
タイッツー is definitely taittsuu. Twitter doesn't seem to use a katakana spelling. Your proposal of "tsuittaa" would be ツイッター. Same katakana but different order. There would be no reason to read Japanese from right to left. Might be an intentional pun though.