Funny to think:
Newsgroups and early mailing lists were all plaintext, so people came up with conventions like using asterisks and underscores.
Web forums were able to display rich text, and they used markup to implement it.
Early social media like LiveJournal and such supported rich text via raw HTML
Twitter was designed to fit in an SMS message, so it was plain text.
And everyone went back to using asterisks and underscores. Plus abusing Unicode lookalike chars.