Once done, go to the Settings menu > PSN > Link with other Services. Select Facebook from the list of services and then select Find Friends. Tick the Facebook friend searches checkbox located below Choose who can see your real name and profile pictures in Players You May Know and select Confirm.
Your console will then update the Players you may know list—leave it to work.
N.B. Only contacts who have already linked their Facebook profile to the PSN will appear in the list.
Select the contacts you'd like to add, create your greeting message, and select Send Friends Requests.