I posted several months ago on how to tag or mention a friend to your comments or status in Facebook. I blog about it to aware the update of Facebook.
Now, Google Plus exist and might be the reason to lose the popularity of
Facebook. You can also mention or tag a circles (google plus) or friend
(facebook) in your status or comments.
In Facebook, you need to add or type @(at) sign then type the name of
your friend. When you type @ it will automatically show the list of
your friends you can mention with.
In Google Plus, you need to add @(at) or +(plus) sign then type the first letter of the circle name to be mention. As I try it, all users of the Google plus or page in Google can be mentioned, even if you're not friends.
I will just update if I'm wrong about this mention features in Google Plus. Happy mentioning to your circles. :p