You can add your Facebook friends as friends in Unkilled. To do so, first, make sure that you are logged in with your Facebook account (tap on settings and there is a Facebook icon). After that, tap on the plus sign at the bottom of the screen and select friends to invite.
Your friend also has to have Unkilled installed on their phone and has to have their Facebook profile connected. You should then see them in the friend bar at the bottom of the screen.
You can chat with your friends and compare medals. To play a duel or a co-op match with a friend, you need to set up a custom game with them.