Friend stones (also known as f-stones) are one of the three types of currency that players use in Monster Life, along with gold and crystals. Friend stones aren't as easy to collect as gold is, but they're only difficult to earn if you don't have friends (referred to as "Neighbors") in the game. Friend stones are used to purchase Monsters, Boosters, and Attractions in the game, and are also used to boost monster stats in battle.
The easiest ways of earning friend stones are by interacting with your neighbors, but you can also earn them in smaller amounts by interacting with random players. Players are awarded friend stones in varying amounts when they do any of the following things:
Adding a new neighbor to your ranch
Liking a neighbor's ranch
Having a neighbor like your ranch
Petting/Tapping/Shaking monsters on a neighbor's ranch five times in a visit
Having a neighbor Pet/Tap/Shake monsters on your ranch five times in a visit
Sending gifts to neighbors
Receiving gifts from neighbors
Liking a random ranch
Petting/Tapping/Shaking monsters on a random ranch five times in a visit
Winning a pack of friend stones in the Lottery, the amount depends on the player's level.
With enough neighbors, earning friend stones isn't difficult, however they can also be purchased with real money. By clicking on the plus sign next to your currency displays on your home screen, you are taken to a screen where you can purchase Crystal, Gold, and Friend Stones from your device.