The news of the SEC investigation seems to be the cause behind the stock's recent drop from $307/share to $270/share, though; if I'm doing the math right, that 12% translates to ~$6.3B worth of value. Kylie Jenner's tweet "only" erased $1.3B of stock value. So, I think Elon has her beat.
That still doesn't seem to be enough to be the most expensive tweet, though. That honor seems to be this tweet[1], where a finance firm "Selerity" tweeted Twitter's missed earnings report prior to Twitter really announcing it, costing Twitter ~25% or $8B. (The earnings report was uploaded early to the investor relation's page on Nasdaq, apparently, and it showed that Twitter missed its earnings.)
Selerity tweeted content that was already posted on Twitter's IR site. That $8B was likely to evaporate the following day anyway. (Weather forecasters who predict a hurricane don't cause the billions in hurricane losses either.)
Social media gaffes of the "optional" variety (Kylie, Elon, Roseanne) are a different category IMO.
That still doesn't seem to be enough to be the most expensive tweet, though. That honor seems to be this tweet[1], where a finance firm "Selerity" tweeted Twitter's missed earnings report prior to Twitter really announcing it, costing Twitter ~25% or $8B. (The earnings report was uploaded early to the investor relation's page on Nasdaq, apparently, and it showed that Twitter missed its earnings.)
[1]: https://www.bbc.com/news/technology-32511932