You can tag or mention any Facebook page or profile from the reply editor for Facebook message posts, comments, or replies.

To tag or mention a Facebook page or profile from the reply editor, you can select any message type other than a private message. For example, a brand might choose a reply message type to reply to a comment by tagging or mentioning a desired account in the reply editor.

You can also select any Facebook comment or reply (private messages are not supported) from the message stream on the left pane. While replying or commenting in the text box, type "@" followed by the name of the page (for example, "@Orange"). The pages will appear in a drop-down menu that you can select from.