You get a bonus for inviting your friends and then they have to invite their friends.
"There are levels of benefit for getting more and more friends hooked (to a point, I think; I've never done it)."
"adding friends is a mandatory quest, and interacting with friends enough times earns you exclusive rewards."
"there is (or used to be) a feature that allows you to gain rewards for inviting a friend and getting them to level 10 (which in itself costs money, unless given free membership at the time of creating the account)."