Facebook adding more realistic and naturalism in let get social techinque,
so the parameters decided for your social scope by them may be preventing you to add friend ..
They Moving Towards To create a Artificial Social Media Intelligence.
Some Co-relation should be identified by Facebook system to allow you to do above..
* Peoples are Friend Of friend ..or Mutual Friend of friend..
* Common Interest may be a big clue
* Country specific peoples may be considered
* Peoples which joined same groups and community
* Peoples dicussing on same topic they can be expected as Friends
* Same profession, age or SEX
* Same School colleges or Others
So many data Interpretions may be perform before you to prevent from Friend add feature for a particular profiles.
And you get a message like this :
“Do you know this user personally? To prevent misuse of Facebook, this request can't be sent. To learn more, please visit the Help Center. If you believe you're seeing this message in error, you can report the problem.” : - Facebook Message