Info from The Official Mafia Wars blog:
The Mafia Wars Valentine's Day Event is coming your way! For this special event, you'll be working on collecting Broken Hearts that'll randomly drop when you do jobs, fight, or rob your opponents. When a heart drops from jobs, fights, or robbing, you'll also get a chance to get a mini-game where you can win up to 5 more hearts! The Broken Hearts will also be giftable from your free gifts page, so when the event is underway, be sure to be generous and share your hearts with your mafia so they'll return the favor. Something important to keep in mind is that the more base energy you have, the harder the job you should select to get your Broken Heart drops - so if you have 1000 energy, doing 2 energy jobs in New York or Italy won't get you heart drops.
The goal here is to collect as many Broken Hearts as you can get your hands on - you'll then get to pick from 4 loot items to redeem the hearts for. The more valuable the loot item, the more hearts it'll cost you. If you're really lucky, you'll can also have a chance to loot the rare Golden Hearts, which are equivalent to 5 Broken Hearts. You never know where one will drop or if one might appear when you accept a gift, so be on the lookout.