Before now, Facebook let users tag their friends in updates and comments by simply including the "@" symbol before they started typing the name. But that was unintuitive for many. Last summer, the company began testing auto-suggest for tagging friends, but the feature later disappeared. It began predicting names after three letters and was likely more annoying than useful. The difference now, it would seem, is that you need to type your friend's full first name, properly capitalized, before the list drops down. For example, "John" shows all my friends named John, but "john" doesn't.
via readwriteweb.com