You get a bonus for inviting your friends and then they have to invite their friends.
"We have to share the game in fb once for every weeks with a reward of 50 ingots."
"Having others "like" your dorm gets you friend points, which are exchanged for cosmetic items. Some events require you get a certain number of "likes"."
"New players can enter your recruitment code, which will earn you rewards. A certain number of players must be recruited in order to earn exclusive dragons."
"Minor in-game currency bonus for friends who join or click on the player's link"