as per the documentation over here
I am able to post on behalf of the user of my application and as well tag user's friends by using app access token.
but I have problem with mention. As said in documentation my message formate is
in facebook it look's same
Testing @[MyFriendsFacebookId] , it is supposed to be
Testing [Name of my friend with the link to his ACCOUNT]
you can't mention friends in posts via app as far as I know. There's a discussion here on SO on the same topic. I'll search it for you.