What is a "Handle" on Social Media?
A handle is a text code like a username that uniquely identifies a user. On social media, handles typically appear on the URL of the user's profile page. For example, in the URL https://www.youtube.com/@virtualcuriosities, which is for a Youtube channel, the text code virtualcuriosities is the handle.
Handles are often used to mention users on social media by writing @ before their handle in a text post.
Why Are Handles Not Called Usernames?
I'm not sure, but one reason could be that the term "username" implies immutability, i.e. you can change your handle, you can change your display name, you can change your URL, but you can't change your username.
For example, on Bluesky, you can change your handle after creating an account. [How?]
While on Reddit, it's not possible to change your username after creating an account.
The handle on Bluesky can be used to log in, just like the username on Reddit.
On Tumblr, things are a bit more complicated because a single user may have multiple profiles (called blogs). In this case, the handle of the primary blog is used as the username of the user for logging in.
