The term "X" is already in common usage: primarily as a letter. If I said "I saw on X" out loud, most people would have no idea what I was talking about;
"They saw on _a letter_?"; "Are they saying some kind of algebra?"; "Did they forget where they saw it and X is a placeholder?"; etc.
You could disambiguate by saying "X Dot Com", but that's longer and more awkward to say than Twitter.
Essentially, "X" as a name for Twitter is a confusing, poorly-forced meme that most folks don't care to entertain.