I am sure that the actors are aware of the pros and cons of setting up a facebook account for all to see. However, I do agree with musicalsfan, that it is far more cheaper and easier to just update your facebook status then it is to have to pay someone to update your website.
I know that there are actors out there who have two facebook accounts. One that is strictly personal and one that is used for fans to have access to.
I know that if I were a Broadway actor I would most likely prefer to use Facebook as a way of updating fans about what is going on rather then having to spend the money and have to pay someone to do that which I can do myself for free via a different method. But, that is just me.
"If you try to shag my husband while I am still alive, I will shove the art of motorcycle maintenance up your rancid little Cu**. That's a good dear"
Tom Stoppard's Rock N Roll