Lives received from Facebook friends are notifications sent in Facebook. They're held in your profile and delivered into the game once per notification.
This means that if you update Facebook, change your profile, reinstall the Facebook app or even update its app software, those Facebook notifications may get lost, as they've been delivered and cleared from the queue. While it is possible to store in-game messages from your friends, due to these factors outside our control, we cannot guarantee that they will remain on your device indefinitely.
Lives can also be lost through an update to the game, your own device or by restarting your device, which clears a certain amount of memory, depending on your device. We therefore recommend limiting the number of messages and lives that you save; a full mailbox can also affect the functionality of the game.
Don't forget that you can always request plenty more lives at any time from your friends! Also if you use a King account instead of Facebook, you will not encounter this issue.