I think that message looks like a misguided sort of advert. A lot of people have deleted their accounts recently because of the new privacy sham and I think that facebook may just be sending that to everyone without an account, as many people have deactivated them.
In answer to what might you do with a facebook account: personally, I just use it minimally and quite enjoy it. Just ask close friends and family if they want to join with you, set everything to "friends only" and you can share quick messages on your status....can be as simple as what you did today, or comments about a favourite radio proramme. Send birthday messages, so it doesn't show if you forgot (as facebook reminds you).
The best feature is photo sharing: I have just had pics of my nieces' camping weekend and my friend's trip on the Thames. Wedding, holiday and anyone's new baby photos can quickly be shared.
I know other people like to play games with their friends, such as scrabble but I haven't done this as I think it would take me over!
So those are my reasons for facebooking and I only check it a couple time weekly. other people use it for invitations etc.