You just have to send your friend's Facebook ID as a parameter under the key "target_id".
OLD SDK: set the FBStream dialog's targetId property.
NEW SDK: set a parameter under the key @"target_id" on the parameters dictionary (when invoking dialog:andParams:andDelegate: on the Facebook object).
Here you have a sample post using the new sdk (the one that uses graph api):
NSMutableDictionary* params = [NSMutableDictionary dictionary];
[params setObject:@"Some text" forKey:@"user_message_prompt"];
[params setObject:@"another text" forKey:@"action_links"];
[params setObject:@"Yet another text" forKey:@"attachment"];
[params setObject:@"SOME FACEBOOK ID" forKey:@"target_id"];
//At some point you need to create the following Facebook instance
[facebook dialog: @"stream.publish"
andParams: params
andDelegate: self];
I haven't actually tested this code, but this is a pretty straight-forward thing to once you have already posted a message to your wall.
Hope this helps. Cheers!
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