Thanks, that has explained what I couldn't understand. I couldn't see any point in linking to my facebook page from my site for the same reasons that you give, i.e. I would want traffic to come from facebook, not go to it.
I installed some boxes from "addthis" to my site but took them off because I couldn't see them doing anything. Then I put them back because I thought I may as well have them as not.
So what you are saying is thatall I can really hope for is that people will click on those boxes to show they "like" my site. It all seems a bit silly to me