If they, or someone else close to you, doesn't inform you of a major life event like that outside of Facebook, maybe those people aren't anywhere near as close as you believe them to be.
I was under the impression that having loose ties is important for various reasons in life, assuming they aren't taking up significant mindshare with non-essential things. (Then it's not really a 'loose tie')
Would rather offer congrats in person than write a generic message on their social media wall.