When I am commenting on other posts, I use @ to ping other users. Sometimes when I type @ and the user's name, it auto-generates their full name if I type in the first few letters. But other times it does not, and I have to type the full name. I don't know if they are actually being pinged.
See also: How do comment @replies work?
If you're commenting on a question or an answer post, the poster is automatically notified and the @ method is redundant.
If a user (including the question/answer poster) comments on the post. You will be able to use the @ method to ping the user and the username will be auto-generated.
You will be unable to use the @ method to ping users that have not left a comment on a post.
If you're posting in chat, the @ method works on those that have recently left a message in chat.
You can also reply to a specific message by clicking on the option tab on the left of the message and selecting "reply to this message" Or use the shortcut on the right of the message.