Great summary from Tracy Sestilii on Socialstrand about how the different social networks function re: tagging people.
How do you tag on specific social networks?
“@” plus the user name of someone you are connected with OR a brand page you’ve “liked”. For example: @Tracy Sestili Writer would link to my Facebook page if we are connected. You cannot, however, tag a brand, a person, or a place that you are not connected with, friends with, or have previously “liked”.