If you aren't familiar with Stocktwits, ... it was they who came up with the Cashtag notation, in 2008.

So when you see $AAPL, for instance, that "$" prefix indicates it is a Cash tag, and that method of tagging currencies and equities took off on Stocktwits long before [Twitter eventually recognized that as a stock or currency symbol:

https://twitter.com/twitter/status/230098997010911233 <-- Jul 30, 2012

Here's more on Cash tag and its origin at Stocktwits.

‘Cashtag’ Ticker Symbols On Twitter Now Send Users To Search Results https://martech.org/cashtag-ticker-symbols-on-twitter-now-send-users-to-search-results/