From PokéHeroes Wiki
Send lovely cards
Until February 14th, you can collect Sweet Hearts by showing love to your Pokémon (interacting) and by buying fluffy Dream World Plushies for your friends. The collected Sweet Hearts can then be used to buy Valentine's Cards. Everyone who sends out a card to at least three different players will receive a brandnew Event Pokémon on February 15th!
And as a special bonus: The more cards you send out, the higher is your chance to receive a Shiny Event!