User mentioning only works on Twitter and Mastodon.
If you want to mention someone on a Twitter tweet, simply include @name in your social message.
If you want to mention someone on a Mastodon toot, you need to keep in mind that a person’s username is like an e-mail account. That is, it’s @name@instance. If you mention someone on Mastodon only by @name, the software might assume the person is on the same instance your profile is, so if the user is not in your instance, the mention might result in a broken link or bring you to somebody with the same name on your instance. Therefore, we strongly encourage you to use the full @name@instance mention format.