You get a bonus for inviting your friends and then they have to invite their friends.
"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"