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”.
You don’t tag on Twitter, you mention a person or brand by simply including their Twitter handle in a tweet. For example: How to tag people and brands on social networks via @socialstrand @tracysestili.
Using a “+” or “@” symbol before a person or brand’s name. Unlike Facebook, you do not need to be directly connected to a person, brand, or place to tag them in a post or photo.
There is no official way to tag someone on LinkedIn. However, many people use the “@” symbol when they are referring or replying to someone in a group discussion, for example.
You can use the “@” symbol and select someone you know or you can also tag a person in their own post even if you are not connected to/following them.
There is no such thing as tagging a person or a brand on YouTube, but you can add “tags” which are keywords for your video that are used for when someone searches YouTube.