Valentine's Event 2016
If you were able to send a card to three (3) different users, you would be able to obtain a brand new Event Pokemon on February 14th.
Every Valentine's Card cost 50 Sweet Hearts each to send. You have the option of attaching a message or anonymously sending the Card, though neither of the two are required.
Sweet Hearts are obtained by interacting or by sending a Dream World Plushie to another user.
Once obtained, they could only be used to send Valentine's Cards to other users. Sweet Hearts are not recycled to the following year's Valentine's Event.