Call me old-fashioned, but I'm of the opinion that if you've become accustomed to sending messages to people on special occasions via Facebook, then you've gotten lazy!
If you have a real connection with someone, or you are building a connection to someone, then an occasion like a birthday, wedding, engagement, or birth merits either a paper card (trips to the stationary store can be fun! and you can even pick up cards at your corner drug store!) or a phone call.
A Facebook congratulations, especially if you know someone well, should be an "extra" on top of the more personal form of contact.
Think about how much personal notes and calls mean to you, and consider making them a part of your personal practice.