What's up on Facebook when a friend sends you one of those postings that you have to share with multiple other friends, Is it a form of scam or just good fun?

Depends on the post.

If it's a "send this and you'll have good/bad luck" ... It's a scam, in that sending it or not doesn't really change anything.

That's why I don't allow anybody else to post to my "wall".  Plus, there's too many idiots reposting stories, without bothering to do even a simple SNOPES search about their truth.