Today in 1536, Florentine merchant Francesco Lapi employed the @ sign as an indication of a measure of weight or volume, an early recorded evidence of such use.
In late 1971, Ray Tomlinson used the sign to distinguish local email from network email on the nascent internet, to indicate that the user was “at” some other host rather than being local. Later, some social media platforms have used @ to designate a specific user.