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)."
"Breeding Peridots is much easier with linked friend accounts. There’s a lot of pressure to friend strange in order to breed your pets"
"At some point the announced a limited time offer for inviting back old players who left the game."