I sent somebody a message on fb about somebody passing. They said sorry I just saw your message. How show should I respond ?

Unless you have additional information to transfer, why respond?  Mission accomplished.

Well, that's a strange way to pass on the news. But there's no need to respond now unless you have some more information. For example, if you want to tell them when the funeral or memorial service is, you could do that. 

You don't need to respond. You sent them the message, they acknowledged it, job done.

pls pray for him/her for GOD.