When friends visit your Springfield they tap on a building associated with a character. That gives the character a Valentine. For example, if they tap on Bart’s treehouse, Bart gets a Valentine.
The best thing about this is that as you tap on any Valentine left by a friend, you can see the name of the friend that gave your character the Valentine – and that means that you can see which friends are visiting you regularly.
Update: Two more points that are clear from today:
1. Your characters don’t need to be doing tasks to get Valentines.
2. You don’t have to clear your Valentines for a particular character to get new ones. Multiple Valentines are possible, which is great news if you have other things to do!
Further update: Just tested it out with my son, who has the game and doesn’t play much. When I went in on my main game, there was only a BE MINE card over his Krusty Burger. When I asked him to go into his game, all his characters were walking around with two Valentines over their heads, sent by both my IDs. The only one who didn’t was Krusty, who I’d missed for some reason every time.
I got him to collect his Valentines, then went into my main game again. Sure enough, every character had a card over its head.
So – your friend must collect a Valentine you sent to that character before you can send another Valentine to the same character. However, different people can send a Valentine to the character that received a Valentine from you, whether or not that character is busy on a task.